There are two ways that you can get to heads up play in a typical poker tournament. Either the poker tournament itself is particularly designed with heads up fashion, with two people putting up their money and the winner taking it all minus the fee charged by the tournament host or alternatively you have been in a sit n go or a multi-table poker tournament and have made your way through the field and down to the final two players. Either way, you are going to have to dramatically change the way you play the poker game if you hope to survive in heads up play and therefore there are a number of things you need to think about when determining the best way to play when heads up.
While you may have heard the word initiative thrown around in some of the poker books or poker articles you have read regarding poker tournaments, chances are you never really gave it that much thought and when looking at the context of a full table in a tournament, you would have been right not to give initiative too much thought. Playing tight but aggressive means taking the initiative only when you have a really strong position and that type of poker play can get you through the early, middle and even some of the late stages of a poker tournament.
However, when you get to heads up you need to take the initiative all the time; not just when you have a strong position. The person that plays strong and hard in heads up is the person that is the aggressor and that person is usually the person that wins the heads up battle. You can even overcome a gigantic chip disadvantage in heads up play if you play hard and aggressive poker and manage to frighten your enemy into tightening up and playing more passively.
While initiative is the thing you should be thinking about most when you get down to heads up play, another thing you need to be thinking about are the relative sizes of yours and your opponent's stack. These are the main weapons that you both have and they are the things you are trying to take from each other to win, so naturally giving a glance towards your stack needs to be something you do all the time. If you have the big stack, then taking coin-flip all in situations is classically what you would do, because your opponent is out if they lose the coin-flip and you're still kicking if you lose it.
However, if the result of you taking the initiative is your larger-stacked opponent playing back at you aggressively, you need to pause and consider changing gears. You want to be aggressive in heads up play, but being aggressive is not the same thing as being reckless. If you get played back at and your holdings are absolute crap, then by all means go ahead and fold. If this happens two or three times in a row, then you might need to consider changing your betting strategy and easing up on the aggression in an effort to change gears and throw your opponent off balance. Aggression first, but stack considerations might result in a change of aggression later on.