First things first.

You are not a tank. Don't try to be.

Riven is an AD based champion who uses lots of mobility. I'm not going to describe the entire champion, because chances are you already know that and are looking to do it right.Think of yourself like a Butterfly with a razorblade. Flutter around, pop in, slice that mofo, pop out.

:Edit:
I address my reasoning behind not using a Bloodthirster and taking Infinity Edge, As well as why you shouldn't start with Dorans Blade, and why I feel Warmog+Amtas isn't as useful as my build.

This build DOES have magic resist, you need to read the items section! Just because its not put in the items above doesn't mean its not there, I say it about 5 times in this guide, and people still don't read.

The Thornmail in the Item Build Above isn't mandatory at that time,Its more of a placeholder for a defensive item.If you read the Item Section below, I explain when you should buy a defensive item.

Riven has one of the lowest base defenses in the game, and one of the lowest growth per level defensively as well.

Heres a chart I made to display this.

Lets compare Riven to the "squishy" melee assassins.
Base Armor
Riven=15 Gains 3.1 per level =18.1 at level 1
Shaco=15 Gains 3.5 per level =18.5 at level 1
Talon=15 Gains 3.5 per level =18.5 at level 1
Xin Zhao=16.2 Gains 3.7 per level =19.9 at level 1
Akali=13.5 Gains 3.5 per level=17 at level 1
Evelyn=12.5 Gains 4 per level =16.5 at level 1
Irelia=15 Gains 3.75 per level=18.75 at level 1
Katarina=14.75 Gains 4 per level=18.75 at level 1
Nocturne=17 Gains 3.5 per level= 20.5 at level 1
Poppy=18 Gains 4 per level=22 at level 1

Even with her buff, she still has the defense of a squishy assassin, but at least it isn't lower than them anymore.
My runes and Masteries correct her defense issue, and the rest falls into place.
This build is about fixing her early game, and making her tough enough to stay in lane consistantly, and never giving the enemy a good chance to push you out. You will take very little damage from pokes, and that will regen in only about 8 seconds.

Don't be afraid to pick up an hp pot or two if your lane is very aggressive or very punishing.

Base Stats
Attack Speed per level increased to 3.5 from 2.9
Base Armor increased to 15 from 12
Broken Wings bonus attack damage ratio increased to 0.7 from 0.6.
Ki Shout cast time decreased to 0.25 from 0.5
Valor
Shield increased to 60/90/120/150/180 from 40/70/100/130/160.
Cooldown decreased to 10/9/8/7/6 from 11/10/9/8/7.
Wind Slash
Cast time decreased to 0.25 from 0.5
Missile speed increased by 10% to 2200 from 2000.
More attack speed-Not too big a deal, but can't hurt.
Base Armor Increase- Perfect. Exactly what she needed.
Broken Wing Damage Bonus-Not bad, I never used it too much as a direct attack, but I'll take it.
Ki Shout Cast Time reduction-Excellent. This will make it easier to get it off without missing due to them moving an inch away.
Valor- Stronger Shield, lower cooldown =Win all the way
Wind Slash-Faster to get off,faster to travel.

These help a lot of her small quirks, but doesn't change my build nonetheless, its just going to make things easier.

In the particular game highlighted, the enemies had some strong AP users,so I picked up force of nature instead of Thornmail.

Below are some more games I played today, I felt I could provide some more examples.

The bottom three wins my team had me solo top, so I was pinned down mostly and was unable to become a killing machine like I so desired.They were 20 minute surrenders for them, they couldn't push my lane and the rest of my team ganked them every time they tried. The loss was a long but good match, most of the deaths came from saving turrets and protecting the home base crystal.
The top two speak for themselves.

Rivens Damage comes from her passive. It has a 50% bonus AD damage bonus on it, so half of your bonus AD is added to every charged auto attack.Your Q doesnt kill them, your auto attacks between them do. If you spam Q three times when attacking, you aren't doing it right.

Broken Wings is not to attack them with. Its to give you a charge of your passive, and move up to the enemy. Right click your target to set yourself on them, then hover your mouse over them and hit Q. You will leap at them,wait til you auto attack, then leap again, auto attack,leap the final time,auto attack, ki burst, dash back away from them. If they have a partner hitting you this whole time, use your judgement in when to dash away, or even to use the remainder of your broken wings to jump away.
::Edit::The only time I really use Broken Wings to attack, is when I'm fighting someone like Teemo who blinded me, and my auto attacks will just miss.Another example is if an Akali is hiding in her shroud near death, its useful to use it to cleave around to get her.

When running or chasing with broken wings, you need to keep clicking ahead of your target if you don't have your mouse over them You can also use it to move around the map quicker. In between Broken wings, she will stop, so you need to keep yourself moving forward as you use it. She has no mana, just cooldowns, so don't be afraid to use your abilities to move around the map quicker. I can catch a nidalee when I chase her.

The Dorans Blade gives you 10 AD, 100 hp, and 3% lifesteal. The lifesteal is useless, as Riven has a very slow base attack speed, and usually has to sit back and move in to last hit a minion. You don't get to sit there auto attacking nonstop to get hp back. It doesn't help you stay in lane longer, you will still have to b to heal up.
The Dorans Shield will regen your hp combined with your runes,very quick early game, which brings you back to full health very fast. It gives you 10 armor, 120 hp, and 8 hp per 5 seconds. You can jump in,hit them, and get back, maybe losing 1/4 of your hp in the process, and then over 20 seconds, get it all back.

Dorans Shield lets you stay in lane longer, lets you recover from poking easily, and lets you farm more effectively than a Dorans blade would.

:Edit: I was soloing top against a caitlyn and a leona, and with my regen from the Dorans Shield and the runes, they never sent me back until a master yi came up from mid about 16 minutes in and the three of them tried to dive me, and I had to use Broken Wings and Valor to escape all the way back to my base.
It all ended well, our Alistar came up and killed the caitlyn, and our Jungle Cho ate the Master Yi.The Leona tried to get the weakened cho and got greedy, and got headbutted into the turret courtesy of alistar. I got two assists because I used Ki shout and tagged the caitlyn and the master yi. Laning against a huge poker like Caitlyn just isnt possible when you use a Dorans blade, since your slow auto attack wont let you benefit from the pitiful 3% lifesteal, and you will likely be forced back behind your turret, and unable to auto attack freely.

Riven has freaky high base HP Regen. Most champs have 4-7 per 5, and get from .5 to .7 per level. Riven gets 12 hp per 5, and gets +1 per level. Early game, this can be a huge advantage if you do something about it, such as... starting with Dorans Shield. You could also start with a regrowth pendent, but I prefer the Shield for the armor and hp it gives you as well.

I don't like warmogs and impaler, simply because in all the time it takes to put that combo together, you have no bonus attack damage and are doing minimal damage with your passive.
Those two cost you 5355.And thats not counting boots, or starter item. With 5402, a tiny amount more, you could buy Dorans Shield, Brutalizer,Ionian Boots, a BF Sword and Ruby Crystal and Long sword for your Phage. With my runes, you can survive the early game, and with the items have enough damage to kill people. You get more hp later on from Frozen Mallet, and if you were inclined, you could make the impaler your last item when you sell your Dorans Shield. I personally like to use the last slot for something more defensive to help me get in and out since my damage is usually fine, but you could always pick up atmas at the end.
If you don't build proper defense on riven with runes, then getting a warmogs doesnt mean much. She has 15 armor at level 1, and with the atma, you will get a little more, but still less than proper runes, and your hp will still disappear pretty fast.

Putting a small marshmallow in a blender leads to it being destroyed.Put a large Marshmallow in a blender, its still gonna get destroyed.Put a medium sized rock in a blender, and you break that thing. Thats right. I called hp stacking Rivens

BIG MARSHMALLOWS!

Riven is a high risk champion

and by jumping in to the middle of team fight, you risk all of your stacks of Bloodthirster each and every time. Getting any of the "stacks" items, such as Leviathan, Sword of the Occult, or a Bloodthirster is adding unneccessary risk to your stats.

"But the lifesteal is really helpful!"
Yes, but you are auto attacking between abilities, then jumping out.With no stacks of Bloodthirster, the 15% will only heal you for maybe 250 hp late game with all of your hits combined before you jump out of the fight. Your slow attack speed means you can't sit there outhealing a Master Yi using highlander on you, auto attacking your brains out.You need to maintain 25 minion kills at least on your Bloodthirster to give the damage Infinity Edge does, and IE has the chance for a nice crit with bonus damage. It gives you a higher base damage, and gives more bang for your buck for only 850 more gold.
I would rather have a 42%(Infinity Edge+Mastery+Ghostblade) chance to crit with a passive enhanced auto attack for 800 against a carry, then to hit for 350 and then heal for 75 per auto attack.

So far, i have found that you should avoid laning against a Rammus,Alistar, Singed, or a good Ezreal.
You can't really hurt Rammus, and chasing a Singed to smack him once or twice isn't worth the massive poison damage you will take.
Ezreal isn't always a problem, but I have laned against one who saved his arcane shift for whenever I got near him, so I could never really land hits on him. He has more distance on his evasive skill, so a good one will just poke you endlessly.
Alistar can mess up your Broken wings, or your Valor by headbutting you in random directions, or knocking you up and stunning you.He nullifies your excellent mobility if you aren't careful.

If you end up laning with against one of these champs, you had best switch lane with someone better suited to fighting them, or hope you have a really damn good lane partner.

:Edit: Also avoid laning against a Blitzcrank, he can yank you(of course) out of safety, knock you up in the air, and then silence you with his ulti.During that entire period, you can't move, cant use any abilities, and thus, cannot escape.If he has a strong lane partner, you will probably die if you get grabbed once, even with the armor and magic resist from my runes and masteries.

For Marks I picked Armor Pen because your auto attacks in between your ability needs to hit hard.
For Seal, flat Armor, because her base Armor is the lowest of any melee assassin, and she doesn't have the burst they do.
For Glyphs, flat Magic Resist, because shes quite squishy, and its needed to stay in lane longer.
For her Quints, 2 hp regen per 5, and 1 armor.

With the Dorans Shield you buy at level 1, you will start with 26 hp regen per 5, 48 armor, and 51 magic resist. Any damage you take, you will regenerate rather fast, letting you dance forward to push them back and poke.

The masteries are 22 offense for the extra armor pen and damage, not to mention the 4% cooldown reduction. The 8 in defense are needed to survive of course, and the regen is an extra bonus for your early game with your Dorans Shield.

Dorans Shield is required for my build, as it gets your defense into the sweet spot of 45-51 that Riven requires to not be forced back every time she tries to fight.
Brutalizer is next,for the extra damage, armor pen, and the cooldown reduction is perfect for her. The more you can use your abilities the better.
Ionian Boots of Lucidity are next. the 15% cooldown reduction helps you use your abilities quicker, and a faster broken wings means more mobility, and a faster cd for your dash.
Next, a BF sword is REQUIRED. This is when your damage will start to really become significant.

This marks the core build.I build this every game and I have no issues whatsoever in ***kicking.
Now that the BF sword is done, you can adapt to the match.

The next item you purchase will be your Phage if the match is going well, but if it isn't, then you have to adapt to the match and purchase a Thornmail,a Force of Nature,or a Quicksilver Sash.
If they have a lot of strong AD champs who are kicking your ***, buy a Thornmail.
If its AP champs, buy a Force of Nature.
If its a mordekaiser, malzahar, morgana... one of those champs who put debuffs on you and crush you with them, buy a Quicksilver Sash.
If the game is going well,continue the build as normal.
Purchase Phage, for the extra hp, and occasional slow
Then turn your BF sword into infinity edge, the Phage into Frozen Mallet.
At this point turn your Brutalizer into the Ghostblade.
The last item is always match dependant.Get something defensive based on the enemies strength. So either the Thornmail,Force of Nature, or Quicksilver Sash if you haven't already. If you bought it earlier, create the Frozen Mallet.

You can sell your Dorans Shield once your 6 slots are full for something else, but until that point, you might as well keep it. There isn't any reason to lose the defense if you don't have to.Again I'll say it;Depending on the enemy team, you can sell it and purchase a Thornmail/Force of Nature/Quicksilver Sash,whichever one you didn't buy already, and is most situationally useful.

You get your Broken Wings for mobility, not for damage. The sooner you learn that, the better off you will be. Its the last ability you will put points into, because each rank isn't too strong.

Your Ki Shout is what you max out first, it does great damage, and is good for clearing a bunch of weakened minions, then backing off with Broken wings and Valor. This is excellent for last hitting minions, i like to tap a couple minions with my auto attacks enhanced by my passive, then use Ki Shout to clear like 4 minions in 1 hit.Happy farming.

Valor is what you max second, after Ki Shout is done. The lower cooldown is mainly why, the shield is just a handy side benefit that can sometimes save you from an ignite.Additionally, if you have the rare 1v1 where you and someone else are trying to kill each other stationary, dont be afraid to use this for shield, and for the Rune Blade stack it will add.

Blade of the Exile you level every chance you get of course. The key thing with it is to remember to activate it before you jump into a fight, the 20% damage does wonders. wind slash is a bonus to it,dont think of it as your TRUE ultimate. The real ulti is the damage boost.

Flash is fine, it can help you pursue, or help you escape. The only people who would be able to catch you after you use broken wings three times, and valor and a flash, and then repeat are a Rammus and a Singed, maybe a Vayne.

Ignite is more effective than exhaust in my opinion, since Riven has a really slow attack speed (which is fine, you shouldn't be standing there auto attacking)so during an exhaust, you get minimal damage done. Your Broken wings lets you catch a runner, and the knockback stops them for a moment, and your ki shout stops them for another. Your dash lets you hit them again.The ingite at the end along with a wind slash is usually enough to finish someone off.

Ghost is acceptable if you are a big fan(Which I am), but if your movement speed gets too quick, i believe its more effective to run, than to combo your abilities to escape.

Learning to use your Broken Wings and Valor to escape people is usually more than sufficient. Save the ghost or flash for a real problem, like all cds down and stuck in nunu's ulti, or flashing over a wall.

This is all you really need to know about Riven, shes a great champion with low defense. Her focus is getting in, hitting them a few times, and getting back. Then going back in again if your hp is up to it. Use my rune suggestions and start with a Dorans Shield, and you will live long enough to kill your enemies.
If you didn't notice already, I put huge importance on the Dorans Shield, the thing is wonderful for her, since it balances and fixes all of her weaknesses when you also use my runes in her laning phase early game. Of course its less useful later in the match, but thats why you sell it when you need the room for the last item.