If you want to be really successful at a poker tournament, then it is a good idea to divide the tournament into early, middle and late stages. In each stage of the poker tournament, you should have a primary playing style, keeping in mind that you will need to shift gears when it seems like your opponents have you pegged. Here is what a sample strategy might look like.
Early: You are never going to be able to win a poker tournament in the early stages, but you can definitely guarantee defeat. You can guarantee defeat either through making stupid plays that spread your starting stack too thin or alternatively you can guarantee defeat by getting your money in good and then sustaining a bad beat. With all of this in mind, your goal in the early stage should be to survive. Survive the bad beats, survive the coolers and survive anything that might be a consequence of your own bad play.

To that end, you want to play tight and actually reasonably passive as well when you are in the early stages of a large poker tournament. This means that you only call with good premium hands or alternatively with hands that can flop monsters with a lot of other players in the pot. Your goal is to avoid confrontations early on that would involve a large portion of your stack and therefore you want to win small pots or win large pots when you are holding the nuts.

Middle: The middle stage of the poker tournament has seen most of the dangerous landmines exploded already and therefore you will see that players that are still remaining tend to be a bit better, although the luck factor in large tournaments will ensure that some very poor players will have made it this far as well. Your goal during the middle stages of a tournament should be to shift into a tight-aggressive mode, bordering on loose-aggressive when you get close to the money. This is because people tend to tighten up at this stage in hopes of getting into the prize money and you can take advantage of this tendency and scoop a lot of blinds and antes if you are aggressive in your play.

Late: The late tournament is the part of the poker tournament where you want to be on your guard and play the form of poker that you feel most comfortable with. This is because a lot of the people that are still remaining are either people that are running good or people that are the best and in both cases you want to be absolutely sure that you bring your best game in order to play with these people. Bringing your best game is a lot easier to do when you are playing the style that suits you best.