Every time someone would tell me that they used to smoke or that they'd quit, I'd ask them how they managed it. Over time some started smoking again, some didn't, and a pattern emerged.

The critical factors I identified were a public commitment to stop smoking and a definite future date to quit.

Having the date set in advance, a few months away, gives you time to mentally prepare yourself, gets you used to the idea of becoming a non-smoker, and adds a very helpful fallback of "I really want to smoke, but I'm not going to throw away all those months of effort".

Making a public commitment (i.e. telling everyone who'll listen that you'll be quitting on [date]) makes your quitting date a bit more important, meaningful and harder to change your mind about as it approaches. It also increases the cost of recidivism: you'd have to admit you failed.

My doctor was one of the people I told about my intention to quit. He offered to prescribe a course of Zyban and I believe it made a huge difference. Just make sure you find out how long you're supposed to take the Zyban before quitting, and start taking it sufficiently in advance of quitting day.

Good luck!