This is what I did (and everyone has their own way)
--Kept my mouth and hands busy (ate sunflower seeds, chewed gum/pencils/pens/straws)
--walked around when I had a craving (sometimes did some push ups or sit ups)
--complained into a voice recorder. It was both helpful and hilarious when I listened back to it a 10 minutes later
--a couple weeks into quitting I'd trick my brain and tell myself I would allow myself a cigarette at the end of the week but when the end of the week came I'd say I'd allow myself a cigarette at the end of the next week and so on. It gave me a sense of control and something to look forward but when the end of the week came I just repeated the cycle
--read success stories online
--planned out what else to do when I would normally be smoking (work/school breaks, in my car, when other people were smoking, etc) it is important to be ready for those times when you realize, "Oh... i can't smoke right now"
--allowed myself to miss it and still want it but NOT surrender to it
--weighted the pros and cons
Good luck man. It isn't easy but you just have to 1. REALLY want to quit 2. be willing to suffer a bit while you are initially quitting 3. plan what you will do in the situations that will really test you and fill you with temptation. You can do it!!!!!
I quit 5 years ago. I'm 27.
EDIT: Oh and jog! It will both ward off weight gain, be a stress reliever, and help remind you of why you are quitting as you breath longer and better each time you jog.