Well, the 'only' tournament i went to, i prepared by only playing one game in the 3 weeks before hand, with a different army and at a different points level, against a pretty rubbish list...
Next time, i will just face what ever my opponent, who ever that might be (arranged on the night) brings. Although i do plan on using my proper army list
But since other people will probably be going, there will be some 'top tier' armies around.
Whats most important i think, is just knowing what everything in your army does, what it does best, what general tactics you want to be using to best utilise your army. And then it comes down to the individual games, its about using the things you know, and being able to change your tactics appropriately.
The first friendly tournament i did (didn't really count it, wasn't using my Army, and there were only 5-6 players not from our club playing) i played one of the people i didn't know, he had Orks, i looked at his army and thought he had no chance...He won.
I think really it doesn't matter what you face, at a tournament you could come up against anything, you might see a 'rubbish list' and think you've won, but your playing against someone who has been fine tuning there list for some time, and has it perfected. It is probably useful to play different army types that DAaddict laid out already, just so you get the general idea of how to use your army against the most common (and best i suppose) play styles.