For most surgeries, doctors will encourage you to eat healthy, etc, beforehand, but the BIG one (and the one that most people fail to do) is quit smoking. Smoking inhibits the wound healing process, and if you're having a major open procedure -- a tummy tuck or breast reduction -- smoking can really put the damper on healing. I've seen people with post-operative wounds that hadn't closed FOUR MONTHS after surgery, because they refused to give up cigarettes.
Researching doctors is a huge one too, and anyone considering any sort of plastic surgery needs to (a) consult with more than one physician and (b) make sure their surgeon is board-certified in plastic surgery by the ASPS. Anyone with an "MD" after their name can legally perform any surgical procedure, but the guy you want doing your boob job is the one who has done 400 of them, rather than someone who normally works as a podiatrist or a urologist and does some cosmetic work on the side.