I'm the same way when I Dm. Premake the enemies that you're going to use for encounters. If you can't predict which ones will come up, make up some generic ones that you could throw in anywhere. Even if you do all your battles off the top of your head (as I have done myself many times,) don't let on to your PCs that you are. If the battle runs smoothly and everything dies about when it should, they won't care if you had it all written beforehand.

Also, check out the "So you wanna be a Dm" thread, it's stickied. It helped me SO much.