Besides smoking herbal cigarettes, they either do not inhale the smoke (that usually causes a coughing attack for anyone not used to it), or simply suck small amounts of smoke into the mouth (exhaling thick smoke from the mouth is pretty telling). Additionally, some the scene is made and cut differently, to minimize the time when actor actively smokes (so that he or she won't look too fake) - you can see them lighting a cig and then talking keeping it in hand and when have to reply to someone shown on the screen they start the cut exhaling a bit of smoke and then replying. You get the idea.
In case you're interested, cigars and pipes pose even less trouble, as you do not inhale when smoking them (and a bit of smoke is hardly harmful if you do this for a short time required to do some shots) and you can give a good impression of smoking just by gently blowing into the pipe to keep the fire and the smoke on. It looks realistic, because even real smokers sometimes do so to avoid lighting the pipe again and it produces a believable puff of smoke.