recon, recon, recon. I can't stress that enough. If you don't know what they are building how would you know what to build to counter?
And bunkers????!?! This is no compstomp, my man. Maybe one bunker to slow them down. Of course, if you just plain suck..then no amount of advice can help you. If you have a decent build, then work on unit control. Know when to pull back and when not to. That's why Starcraft is a true RTS, you actually will have to think on the spot.
omfg, all you guys are city builders. All I have to say is your best defense is your offense. Just look at this way, if your opponent out manned you with his attacking force then he's simply a better player/builder than you. So, you will lose eventually. But on the other hand if you manage to hold out his attack or he decides not to attack you, then you have a big infantry to attack your enemy's base. Bunker is good for defense only, but your infantry is good for both.