Get into some sport or go do some regular exercise (if you dont already do this), get to the gym, swimming etc, it makes you feel much better and reduces how much you want to fuck yourself up with booze, weed at weekend or at anytime. It makes it harder to train when you have been smoking so if you give up you get better results at the gym, see it as a challenge to yourself, think of it like an achievement "ACHIEVEMENT UNLOCKED - 100 G - Kicked the gyms ass". Also martial arts is good to get into, can help sort your head out
Also if you have a dog, get him out each day for exercise, training etc its a great help, and you will form a better bond with him, rather than "just a pet".
You are talking about it, so you know you dont want to do it, the hard bit is just not thinking about it, spend your time and money on something more enriching for the soul
Clear head, you get more done, fecked head, nothing gets done!