Quit smoking tips!!

Okay I quit smoking cold turkey I did good for a month I didn't have any cravings or mood swings. Well as couple of days ago I started again. Does anyone have any tips? I'm constantly around smokers and I believe that that's what my trigger was. I'm having hard time smoking thay "last" cigarette right now. I don't know why it was so easy for me to have that last cigarette a month ago. Any advice would be helpful.