I wrote this to a friend of mine (in a facebook comment, no less -- I received some WEIRD virtual internet stares), so please ignore anything that seems a bit random. I'll just copy & paste and hope it helps:
Nicotine acts by binding to the acetylcholine receptors on neurons, particularly the beta-2 subtype. Acetylcholine is one of the main neurotransmitters that neurons use to 'communicate' with one another. When smoking, the nicotine mimics the action of acetylcholine and the body responds by growing more acetylcholine receptors on the aforementioned neurons. When someone quits abruptly, the body initiates a panic response, thinking that it's deficient in an essential signaling molecule. Deficiency is obviously an undesirable state, so the cravings for nicotine ensue. With that said, you're only on day 15--it takes a fair period of time for the acetylcholine receptors to normalize. You've probably got somewhere in the range of 20-30% more receptor sites than someone who has never smoked. Fear not though, the amount of receptors in ex-smokers decreases to the level of never-smokers within 6-12 weeks, eliminated the physiological addiction. Here's a study illustrating that: http://www.nida.nih.gov/NIDA_notes/NNvol22N4/Abstinent.html. As far as the psychological addiction goes, there's probably two sides to it. The stimulant effect of nictone--either ignore it as you readjust or maybe introduce a little caffeine (don't OD on espresso) & the repeated movement pattern of just doing something with your fingers and mouth. Go for a hell of a lot of chewing gum or munch on some baby carrots or something. Anyway, if you read all of my nonsensical gibberish, I'm impressed, because that was probably damn boring. haha. I am a dork though, so I figured I'd contribute. Best of luck!