1. Position, Position, Position; most important aspect of tournament poker. Try to play as many hands as possible in position and limit the number of hands you play out of position.
2. Mix it up a little at the start of tournaments. This is the time to see some flops play suited connectors, low pocket pairs, and try to hit a flop big and stack somebody who doesn't like to fold an overpair.
3. With a stack of 15-25bbs you are going to be limited to what you can do. You don't want to be raise/folding very often and you don't want to be calling other players raises with marginal hands. So play tight solid poker.
4. With a stack of 15-25bbs you have a great "restealing stack". What I mean by this is if there is a late position raiser that has been active don't be afraid to reraise all in with marginal holdings. The raiser is going to need a big hand in order to call you and chances are he won't have it!
5. When you are a big stack attack the other big stacks. Tournament poker is all about winning! All the money is in the top 3 places of every tournament so you want to give yourself the best opportunity to win and there is no better way to do that than to attack the other big stacks. Apply some extra pressure to them when you have position and force him/her to a tough decision