Riven is a new character so I'm sure there are alot of builds being tested right now, I threw this one together just last night testing it out and so far it works great for me, but ofcours different people play different styles.

!!!Important note!!!
This build is only based on my experiences from 3v3 so I dont know how well it does in 5v5.

Greater Rune of Desolation - Because armor penetration is always nice especially against tanks.
Greater Seal of Malice - Cause that extra crit chanse is just amazing, I find my self critting often and hard, making other champions buckle easily if you do it right.
Greater Glyph of Celerity - Cooldown reduction means you can use your Q combo more often, always nice for both escaping and attacking.
Greater Quintessence of Desolation - Some extra armor penetration dont hurt.
Greater Quintessence of Malice - Crit, crit and more crits!
Greater Quintessence of Furor - Heavier crit damage makes you stronger ;)

I usually start with Boots of Speed to outrun thos who try to flee or to flee my self, I quickly get the Boots of Mobility cause the extra speed is really nice to dash between the battles if your aid is required somewhere else.

I go after B.F. Sword for the extra damage, I then try to stay alive long enough to get Infinity Edge after that and if not I just buy the pieces till I can afford it cause let's face it, everyone loves the Infinity Edge ;P

Now we have some decent damage but not that good attack speed we need to swing our weapon a bit faster so I go for the Phantom Dancer and start with Zeal for the speed but then get the Vampiric Scepter just so I can jungle and solo the dragon for the extra gold and buffs so I quicker can get up to Phantom Dancer.

After this I go for the Bilgewater Cutlass for the better lifesteal + damage then after that the Hextech Gunblade for even better effect.

Then I go for Madred's Razors for the extra damage on creeps/monsters to even easier jungle and solo them, followed by Madred's Bloodrazor cause now when we hit hard and fast we might aswell hit alittle harder by draining 4% of their maximum health as magic damage ;P

Last but not least we go for Banshee's Veil cause let's face it, blocking thos pesky abilites is always a nice thing to do.

Pros
Heavy Damage in mid-end game.
Nice life steal in mid-end game.
Easy to farm creeps and to jungle early-end game. (Not start)
Decent survivability mid-end game.
Good to flee with if you master the use of Qx3+E+Flash fleeing skill

Cons
Reliable on her Q combo wich does have a 12 second cooldown.
Bit squishy start-mid game. (Atleast for me, might be I usually get 3v1 tho)
Bit on the weaker side start-early game. (Untill you get your B.F. Sword)

What I usually do is use my Qx3 combo then a W for the stun and AoE damage, brings most creeps to the floor. Atleast in the mid-end game.

Same tactic goes for the monsters in the forest with an addition of E for the shield.
Once you've gotten your Vampiric Scepter (About lvl 8-9 if all goes well) you should be able to solo the dragon wich ofcours is always nice

I usually charge in with my allies and do the Qx3 combo ending it with a W for the stun and the moment they start moving I apply Exhaust to make them slower, more often than not in mid-end game that means the death of the enemy champion even when you're alone but especially when you have one or two team members beating them too.

This build is good for farming at pretty much any time after you've gotten two Q and one W attack.

Well for now that is all the tips and advice I can give you, I will flesh it out more as I get more impressions and ideas after I've played her a bit more, but for now I wish you happy hunting in the Twisted Treeline and hope you kill some enemy champions with my build ;)

Good Luck, Summoner!