Ok. Here's a guideline I usually follow when I'm in a tournament. The first third of the tournament I play very tight. This is helpful because there is a lot of manic players that go all in on the first three hands. Unless you have pocket kings or pocket aces, don't call them. Let the maniacs wipe themselves out.
The second third I loosed up my play and start playing even K, 8 or Q, 6. At this stage, you should get a feel of certain player's playing styles.
The last third is the toughest. If you are the big stack, start raising and try to push people to fold.
If you are the middle stack, I usually back out when the two shorter stacks go all in unless I have a good hand. You need to be aggressive when you are the big stack.
If you are the short stack, I usually have this stragedy, "Fold or Go All-in." You're not going to win every all-in but you don't have to. You would be able to "steal the blinds" a few times to build your stack a bit. If someone raises pre-flop, I wouldn't go all-in unless I'm confident I got a good hand to go all-in with. For more on this subject, read the articles we have at cardschat.com. Just clikc the articles tab on the top of the page. Hope this helps.