well this is kind of lame compared to real smoke rings from your mouth, but I know how to make em with a burning incense stick. hold it sticking out straight from your hand, and slowly and smoothly push your arm forward in one fluid gesture, then pull your arm back quickly at the end of the follow-through.

looks very cool on stony occasions when you're bored the smoke rings are very thin and stay together for a while.

p.s. i haven't gotten this technique to work, but i hear you're supposed to form an O with your mouth and "push" the smoke out with your tongue.