The hardest part isn't quiting. The hard part is not starting again. It's easy to quit for a couple months, despite the pain of withdrawal. It's much harder not to want a cigarette after a big meal, or at a party where everyone is smoking, or even just standing around outside waiting for someone. Chewing gum during those moments can really help.

Oh, and if you have one cigarette that is no excuse to give up and go back.