There is a big difference between a tournament that is a rebuy/add-on and a tournament that allows re-entry.
In the first, you can often increase your stack by purchasing additional chips when your stack falls below a certain level, then later add on even more.
For these types of tournaments, it is wise to plan on re-buying at least once and adding on when it becomes available.
If the tournament is the type that permits you to re-enter only if you bust out, then you need to pay attention to blind levels, and what finish position is required to profit