You must play tight when weak and aggressive when strong or when you perceive weakness in your opponents. It is very important to bet when your opponent is afraid he is beat... in this situation the cards don't matter much.
You have to be an opportunistic predator in tournaments. Early on there is often more prey to feed on, so it is important to take out as many as you can. This will give you a big stack that can absorb some bad beats and suckouts which will eventually hit you if you are in a huge field. It will also allow you to play more speculative hands in the early or mid stages of the tourney before the blinds get too big. You have to be in a position to get lucky... and by lucky, I mean playing a hand like suited connectors or a small pocket pair and busting another player's aces or kings. It isn't really luck if it is part of your strategy. The player whose hand gets busted will call you lucky, but if you missed your hand, then you would have folded and lost the minimum. (and why didn't he bet enough to get you out of the hand anyway!?). If you have a big stack, you can do this many times and take advantage of a player who will not normally put you on the hand you have. This is a great way to win huge pots. The only danger here is the temptation to chase. Don't put all of your chips in the pot when you haven't made the hand unless you know your opponent will fold.
Later in the tourney there are fewer weak players and you have to be a lot more selective. You also have to be careful against bad players because they will play ridiculous draws and get lucky against you some of the time. It can cost you a lot of chips or your tourney life when a player calls your "NUT" flush with 2 pair and hits a boat on the river (The word "NUT" is in quotes because it isn't "THE NUTS" if a card that beats you is still in the deck and the river hasn't been dealt). I really hate to hear a guy say "I had the NUT flush and he sucked out on the river!" He really should have said something like... "I had an ace high flush and decided to slowplay it.. only to be beaten on the river by the board pairing and giving him a full house!". Obviously you will beat any one of these players over time, but this situation can happen many times in the same tourney.
If you believe in the numbers, you have to know that if your opponents get it in as a slight dog or worse often enough, eventually they will win a pot. It is almost like the bad players are ganging up on you. Eventually you will take the bad beat and it is best to have the big stack so you don't get busted when the time comes.
