The problem that ex-smokers face regarding cravings are the nicotine receptors in your brain. They take a while to shut down and never disappear completely. Take a non-smoker... he has 'X' amount of nicotine receptors. The first time he smokes, those receptors all fire up... and as he smokes a bit more, the brain makes more receptors. Eventually, depending on how much the person smokes, and for how long, they could wind up with many times as many receptors as a non-smoker. The cravings smokers (or ex-smokers) get are those receptors calling out for their drug. As time goes by, once a person has quit smoking, those receptors slowly begin to go to sleep... but all it takes is one cigarette to wake them all back up again. The theory why ex-smokers get cravings long after they quit is that another group of receptors is going dormant and that craving is one last outcry before they go into hibernation. Eventually, the ex-smoker's level of active receptors returns to the level of someone that never smoked... although they still have many more receptors than the non-smoker, the number of active ones is comparable.
The thing about these nicotine receptors is that they are also the receptors for caffeine... and most other 'ine' stimulants. That's why they never shut down completely, nor would you want them all to shut down. It's just that nicotine makes them 'light up' much brighter than other 'ines'.
All of this has nothing to do with the psychological addiction to smoking... this is purely about the physical addiction to nicotine. The psychological issues that make quitting hard is a completely different problem that can be overcome, if you really want to quit. It's not about 'toughing it out' or 'willpower'...
For reference, I've learned all this in the first 4 weeks of the quit smoking program I'm in... 1 pack/day for the past 18 years, my last cigarette was 11/19 @ 9:30 pm... almost 9 days now.
Quitting smoking isn't easy... nationally, the successful quit rate is only about 20%. The program I'm in (free, by the way) boasts a 60% quit rate.
Anyone that's interested, the Center for Tobacco Control webpage is... http://www.northshorelij.com/body.cfm?id=558
It's on the Nassau/Queens border in NY, a part of the North Shore/Long Island Jewish hospital network.
Wish me luck,