From my first game I decided she will be my main and by my third game I got a pentakill. Riven is the type of champion where you can buy pure offensive items and still not be squishy due to her shield and AOE stun.

Pro's
Constant damage output and not of the light kind.
Very mobile
No mana
No where near squishy
Your ultimate gives you a gert sword which emites an even bigger slash of wind

Cons
Exhaust will ruin your mobility which in turn ruins your day
CC will stop your combo drastically reducing damage output
Riven is kind of item dependent to do damage as the ratios are not the best

Marks of Desolation: This is a no brainer really she is a melee champion who has to be in the thick of it. This will help late game and in some cases cause true damage to squishy's early game.

Seal of Resilience: These will just help you stay alive especially early game against champions like caitlyn who will just outrange you forcing you to get up in her face.

Glyph of Celerity: I was going to put magic resist runes in here but AP caster generally dont do much damage early on and seeing as Riven is cooldown based I thought these would benefit me a lot more.

Quintessences: 1 movement speed rune to help you catch up to your unfortunate target and 2 health regen to help stay in your lane early game simples.

19/11/0 the offensive masteries are obvious more damage and lane sustainability with lifesteal coupled with dorans blade makes it worth it.

Defence wise i fully updrade the beginning armour and magic resist to help early game greatly. And now my favourite...the health regen. Coupled with your runes and beggining lifesteal plus the armour runes riven will become a immovable object but if these fail you then you always have a dash/shield and a stun ;)

Start with a Dorans blade. This will give you some health to help you survive early game some nice damage and some unnoticeable lifesteal.
Next time you go back get Mercurys treads to help with survivability or if you are really roflstomping them and they have no cc get boots of swiftness to help with your already good mobility.

Next you want to start working on your damage so for your first item get a brutalizer. this will give you some nice early game damage and some of that precious cooldown.

Now you want to try and get a bloodthirster as this will see your damage go through the roof and also give you some nice survivability in terms of having to fight to stay alive. After I get this item that is my core done anything after this should be up to you depending on their team.

If you are still roflstomping after the core build I generally go for some more damage such as a infinitys edge or a black cleaver as both add a welcome amount of damage.

But if you find yourself getting killed too quickly by AD carrys get yourself a randuins Omen as this gives you health, armour, cooldown reduction and a nice passive and a very nice active against AD carrys and feeling enemies. If you find the same happening but with AP carrys get yourself a force of nature for a large amount of MR some movement speed and very nice health regen. If their CC heavy get yourself a Banshess veil as this will stop atleast one of those pesky stuns etc.

Oh and dont forget too finish off your brutalizer into a ghostblade as this will help your damage output greatly as well. :D

At first I wanted to max out broken wings but then I realised that Ki Burst does quite a bit of damage AND it stuns in an AOE so its amazing for team fights. It also synchronizes well with valour; dash in, attack, stun, attack, Broken wings, dead.

You only want 1 point in valour to begin with just for that dash BUT if you find yourself getting harrased way too much then you should max out ki burst and valour and forgetting about broken wings as wings is only good if you feel confident and know that you will come out of the attack better off than your opponent.

And of course level up blade of the exile when you get the chance.

Fighting 1v1 can go very well or not very well depending on who your fighting but generally you want to:
Blade of the exile- Valour-attack-Ki burst-attack-broken wings x3- attack x3- Windslash wait for cooldown then repeat.
DO NOT USE WINDSLASH EARLY AS THIS WILL WASTE IT BY THE TIME I USE WINDSLASH THEY ARE FLEEING WITH NO HEALTH. THIS IS A PRIME TIME TO USE IT.

In team fights its pretty much the same but Riven has great disruptive abilities. You want to dash into the middle of them and stun them all and knockback them all as this helps the team greatly. But only do this if you know your team is going to support you or else you will look like a right downer jumping in stunning knocking them back for your team to find no one else is there and you just got your *** kicked.

I choose to use flash and ignite.
Flash has multiple uses it can be used to get away in spectacular ways or to catch up to that fleeing opponent to pop your ignite on or just kill them in the conventional way. Either way its an extremely useful spell and if you are fleeing try flashing over the wall to ensure your getaway (unless they flash with you then run for your life)

Ignite is just the bawz pop it on your opponent to ensure that kill if they flee or if you see a mundo taric etc pop it on them as it reduces healing by 50% which is a right kick in the teeth for mundo.

This is my first guide ever and I hope its helped you in some way but remember don't be afraid to jump in (unless you have brain dead level 1 allies) and don't forget that Riven is very disruptive.

Now enjoy this pentakill :) Play Video (HD if you go to youtube)
Thanks for reading any criticism is welcome :)