If you go broke you are gone
When you play cash games you can always reach in your pockets for more money if you lose what you have in front of you (depending on how deep pockets you have). But in a tournament it is devastating to lose your stack. Once you are broke you are gone, so you should do all you can to avoid it. Try not to commit a lot of chips in close gambles if you do not have to. Even if this is profitable in a cash game, you have to protect your chips more carefully in a tournament.
Early Stages
Early in a tournament the blinds are low in relation to your stack. This means that you can play a lot of hands as long as you get to see the flop cheaply. Small pairs and low suited connectors are good hands to try to limp in. You risk a small portion of your stack for a chance to win a lot if you hit your hand hard.