I like to light a red candle for healing, and may put some amethyst out or hold it in my hand (also for healing). (I figure if something works for physical healing, it probably works for addiction healing as well.) If you feel an urge to indulge in your addiction, repeat the spell to yourself under your breath or in your head. Enjoy and best of luck!
(Replace "_____" with whatever you're trying to quit. I have put in "make your gesture to the God/Goddess" but you don't have to do that if you don't have gestures. A raised hand is fine. I use a crescent moon handshape for the Goddess and the horns for the God.)
Face each direction (East, South, West, North) as you call upon each element in turn.
Powers of air, hear my call:
Let this sick addiction fall.
Powers of fire, hear my plea:
No longer let this control me.
Powers of water, heed me now:
_____ I can do without.
Powers of earth, end this reign:
From _____ I will abstain.
Make your gesture to the Goddess as you say:
Goddess, Lady of the Moon
Make me loyal, make me true
Make your gesture to the God as you say:
God above, Lord of the Hunt
Let me be a _____-free one. (Can also use "let me be a sober one" because the rhythm is better. I used "smoke-free.")
I only wrote this and started using it this morning, but I have no psychological urge to smoke, just the physical withdrawal symptoms, and those are much less than before I did the spell

I realize I have written this from a mainly Wiccan perspective; feel free to leave out the parts about the God or Goddess if you're not comfortable with them. Also feel free to edit any part of this spell so that it suits you - the important thing is for you to be comfortable saying it. No use doing an incantation where you're tripping over the words or feel that the rhythm is off!
Try it and let me know how it works for you!
Blessed Be,
Ether