All the above advice is right.
I am a rare bird though. I started playing large, live, rebuy tournaments ($300r's at Commerce Casino) without rebuying or adding on 1 Unit Max, when I was relatively young in poker in order to get a big prize without spending the large amount normal play in these dictates. I always remember people saying, I can not play in that 300 without 900 in my pocket. Yeah, it takes a patient player to make it happen on 1 unit, but through the years it has stuck with me. It followed me into online poker as well, and I played the 10r (and others) frequently at PS and other sites on 1 and 2 units max and had very good success.
The structure of some rebuys forces you to make rebuys and addons no matter what your preferred style might be (ie rebuys with 1500 starting chips and 10K addon).
Also good advice is NEVER leave the game before the rebuy period is over. Sometimes I will plan on playing only one unit and just leave once I am beat. Sometimes I just don't like the bad-beat I took so even if I planned on taking a rebuy I leave. These days I take the addon more than a few years ago, but I always base it on my chip stack, what the rebuy and addon chips are worth, what it costs v the prizepool, to make my final determination.