How to quit smoking for good.?

ok i used to smoke about 15 cigs a day ..i wanted to quit at first it was hard, but for three weeks now i have only smoked one cig each night before bed,i really think that is just habit. how do i stop smoking this one cig. i still have cravings all day, so i guess i think one isnt so bad..... but id lik eto stop it. at night. any advice. been trying to eat sunflower seeds but its not helping, to keep my hands busy.