I, an avid smoke-ring-blower, feel I need to provide an easier technique to the masses here.

Begin as Gangsta suggests by dropping your jaw as low as possible.
However, the circle you create with you lips needs to be about the diameter of your pinky finger. If you try to move your jaw, you'll force too much of the smoke out all at once.

Instead, it's more of a forcing your tongue forward motion. Start with it laying all of the way to the bottom of your mouth, retracted slightly. When you force it forward, it should remain against the bottom of your mouth, and your cheeks will puff out a bit.

In short order, your should be able to produce five or six "good ones" on a single drag. It's damn near impossible to get a meaningful ring if you inhale, but easily done with a cigarette by simply drawing it into your mouth without the inhalation.