I'm a Riven main (well, on SR) but on Dominion I've never really liked playing her.
The primary reason for this is that Riven loves to get kited around. You can solve this to an extent by running certain summoners (I've heard some love Cleanse/Ghost) and by buying certain equipment, but this just exacerbates the problem: what's so good about Riven is her amazing attack damage scaling.
Lately my Riven builds have started centering around dealing with her Dominion mobility weaknesses. I do feel this is a very real weakness, because you lose out on a lot of AD. All of your abilities become weaker, but the trade-off is that you can force people to fight you, and stay in fights more easily.
I've been using something like:
Prospecter's Blade/boots/2x pots to start, then a very fast Phage, Vamp Scepter, and finished Frozen Mallet. At this point I don't hit very hard, but I can really stick to someone. If I'm still having problems getting kited around (say by someone like Ezreal) I'll go Ghostblade and Warden's Mail before getting Senguine Blade. At some point in the game I want to have Last Whisper, but when I build it depends on how badly I need mobility and sticking power.
My last build looked like: Ghostblade, Merc Treads, Mallet, Senguine, Warden's, and Last Whisper. I was running Ignite and Revive. I like Ignite on Riven because it has awesome synergy with Entropy (good item on Riven if I want to build it) and her ult.
How do you build Riven?