Hi there! I just quit smoking about six weeks ago, and I've been dieting for about five months now. I didn't really plan to quit, in fact, I always said that I'd "never" quit, at least until I graduated college (I'm 22, I've been smoking about a pack a day since I was about 15...I know, bad bad bad). But I'd been doing a lot of cardio and I was always breathing heavily, and I heard about the Couch to 5K running program, and I've always wanted to be able to run. I tried the first week and I was so out of breath; it was SO horrible!! And I went home and I did some serious thinking, and I realized that if I really wanted to lose weight and get in shape, I had to quit smoking.
So I just quit then and there, cold turkey. I didn't tell anyone because I was afraid I might fail, so no one even knew I was trying to quit. I just suffered through the first few HORRIBLE days of craving food and being extremely cranky and really really wanting a cigarette by myself. Whenever I wanted food I just hopped on my treadmill for 15 minutes, and whenever I wanted a cigarette I brushed my teeth or chewed gum or (how sad is this) just sat there holding a butt from the ashtray.
The first few days were the worst, but the first few weeks were pretty rough too. My roommate smokes, and all my friends smoke, so wherever I go, people are smoking. But right away I could tell the difference when I was doing cardio, so that kept me motivated. And I didn't gain a single pound from quitting, I kept losing weight at the same rate I had been while I was still smoking. But I was careful not to replace smoking with eating though, which was really difficult!!
So as far as tips go, I would definitely tell the people around you - get a good support system. Let them know you're about to turn into a cranky screaming monster!

Stock up on healthy snack food (veggies, fruits, etc) and gum. Get rid of all your cigarettes, ashtrays, lighters, anything that reminds you of smoking. Get plenty of sleep and plenty of exercise. Be strong!! You can do it!!