To begin with, it think it depends on your bankroll if you play rebuy- and add-on tournaments. Bankroll management is essential. http://www.thepokerbank.com/strategy...ll-management/
They say you should only have 20 buy-ins in tournaments, but in my opinion it's more safe to have in between 50 and 100 buy-ins.
That being said, when you do play a rebuy/add-on tournaments, you should know that players with a bigger bankroll can take more risks in trying to build a big stack for later on in the game. So always check the rebuy-time and decide on forhand what you're willing to spend on the game. Also look at the prices ... are the lowest prices good enough to rebuy several times? When you have really strong hand and you're being confronted with a reraise, it's advised to go all-in more often, 'cause other players go more all-in with a coin-flip, because the rebuy-option. In general a more aggresive play in the rebuy-period.
Regarding the ADD-ON'S, it is the smartest to always take them. I play on Pokerstars
and I never saw a game where you can take 3 of them. So I'm not sure how to play with several add-on's, but I guess it's the same like with one. The ADD-ON makes that you have more chances to build your stack in the game and you are stronger when all chips go in the middle.
When the rebuy-period ends, you play a little more conservative, like in a normal game. But you can use the conservatism of the others to your advantage to be more agressive once in a while.