ALWAYS buy as many chips as you can as soon as you can.

This is just low-hanging fruit, in my opinion (and most of the top players I know). Buy in, and buy up right away. If you can't afford 2 buy-ins + the add-on, you have no business in the rebuy tournament in the first place. I generally prepare to play 5-7 buy-ins, plus add-on. There is logic behind it:

In the early stages of a rebuy, the action will be fast and furious. Don't show up for a gunfight with one bullet. If and when you get a chance to double up, make sure it's for as much as possible. If you get AA on the first hand, and only have 1500 chips, but there are solid players at your table who've already stacked up, you are still at a disadvantage somewhat. Double 3k to 6k, and now you're in a great spot to take advantage of all the fools.