Get the carpet steam cleaned or use the carpet cleaner-in-a-can and get to work. Also you'll want to spray something like febreze on the headliner since it retains odors quite well. Might be a good idea to go ahead and change the HVAC filter at the same time...don't think smoke odor would infiltrate it but it couldn't hurt to replace it.
Oh and most importantly, quit smoking! :p
