Playing late game

I'm level 30, and not super good at LoL but not too bad either. I've played a few ranked games with my duo buddy. I've noticed that when I play AD carry or AP mid (support, I do just fine) I do really well early-mid game but late game, where it's just constant teamfights, my score and performance always falls off. I try to stay behind the tanks and all, but well, we can't be perfect every time. Does anyone have any tips on continuing to excel late game? If it helps anyone, I play:

AP: Ahri, Vlad, LeBlanc (although I don't really think she's a carry), and planning to buy Xerath

AD: Miss Fortune, Draven, Varus

TL;DR: I do well laning-ganking phase but fall off late game. Got any tips?