Chip leaders popping up in early stages most of the time will loose their stack because of "invisible" feeling, when they think they can call short stack players with marginal holdings. So the main strategy overall should be very simple:
1. Early stages: play very tight, don't call all-ins and don't shove like crazy with hands like KJo A10o etc. If you want to go for the win you need to learn discipline to fold hands like these in positions where you know your not ahead anymore. I would suggest playing just 20% of your hands in these stages, if tournaments are deep I would consider to play even just 10% of my hands. And always remember tight is right.
2. Middle stages: open up your hand range now, about 25-30% I think its totally fine if you know when you're behind and it's not worth running for it. But yet again discipline and patience will pay of.
3. Late stages/Bubble/Final table: they'r is a saying that you should get aggressive and attack the bubble and etc. but a lot of people just forget one thing, that even if your hand is not that important, you're guns are still those two cards, so raising to steal the blinds, you should consider about other players stacks, you're position and bunch of other factors of the game. Just remember that it depends...
And always remember, that you can't win them all. Good luck at the tables.