Rebuy Strategies

This may sound a little fishy...But I'd rather learn from you guys than the hard way. I've played many MTT's in casinos and on stars but am a noob to rebuys and would like to learn a little about the strategy.

How, if at all, do you change your play in the early stages (before the rebuy period ends)?
How many times do you allow yourself to rebuy?
In what situations do you find it adventageous to add on at break?
Any other general advice would be great