Well, Naya is a bunch of beatsticks that just won't stop coming. Yearling, Thoctar...Zoo is a great deck. What you need is either more aggro, or more control. And being that it'll be hard to out-aggro Zoo, more control it is.
Creature removal is key to keeping the game in check. If they can crap out fatties faster than you can get rid of them, then you're done. In type 2 [standard], the best spot removal is Path to Exile. Which, you're lucky, is white.
I also mentioned in your other question that you should use Unsummon as a one-of. I say that simply because it's a geeat situational saver, but it's a not going to win you games. For example, you mentioned having trouble with guys with +1/+1 counters on them. If you send that creature back to their hand, they lose all the +1/+1 counters on them. It's an instant, too, for one blue. So thety could come at you with three big guys. Path to Exile a big one, Unsummon the one with counters, and let 2 or 3 through. Whatever. Chump block it if you want to.
Point is, you just minimized your damage at the low low cost of 2 mana, On THEIR turn. Now they're like, dammit, go. And you're like, hell yeah, draw. And now it's your turn and you have more options while they're open.
It's all about the control cards. Counters work wonders as well, obviously.
Hope this helps a little. =]
Avid MTG tournement player, buddy has a Zoo and I used to run Esper-facts