There are primarily three parts of the body which you must have excellent control of in order to blow smoke rings:

· The Lips

I'll assume most people have decent control of their lips. You will need to keep them in a proper "O" shape in order to form the ring. The size of this will vary depending on the individual, see below for specifics.

· The Diaphragm

The diaphragm is the muscle which controls breathing. You will need to be able to maintain precise control of this muscle, even if the smoke in your lungs is beginning to burn. The diaphragm is one part of a coordinated effort which is required to create an air pulse through your smoke-filled mouth which will cause a ring to form.

· The Epiglottis

The epiglottis is a flap of cartilage which you can pull shut over where your trachea (i.e. wind-pipe) meets your esophagus (the tube that connects your mouth and your stomach). It's primarily used to prevent food/liquids from entering your lungs when swallowing.

Blowing Large Rings (from the lungs)
After filling your lungs with smoke, you will need to contract your diaphragm as if to exhale, but use your epiglottis to only let a small amount of air out. This will fill your mouth with smoke. This should be so slow as if to be subdued, with no real noise produced by the action. Next, with your diaphragm still contracted, let loose a short gasp while dropping your jaw and keeping your lips in an "O" shape approximately 1" - 2" in diameter. The sound of a gasp you are going for is not the typical "aaaaah", it's more of a whispered version of Butt-head's "huh" (as in "Huh huh huh huh, that was cool, Beavis"). You should close your epiglottis immediately after you finish making the sound, which will create a small air pulse through the smoke in your mouth. This will force a small amount of smoke through the "O" shape you should still be making with your lips, creating a smoke ring. If you create a disorderly puff of smoke rather than a ring, chances are the "O" you are forming with your lips is either misshapen or too small; vary the diameter and find the size that works for you.

Blowing Small Rings (from the mouth)
The secret to this method is to push the smoke in your mouth out with your tongue, while making the slightest inaudible puff from your lungs. This will push some of the smoke out, creating a ring. This method requires much more precise control in order not to push too much smoke out, as well as incredible tongue control.

Keys To Success!
Thick smoke is key, especially for blowing large rings. If you are attempting to blow smoke rings with a cigarette, your best bet is to puff off of it quite a bit without inhaling, filling your mouth, at which point you can push small smoke rings out. Cigars will provide a substantially better better source of smoke than cigarettes, and the same applies to herbal sources of smoke, which are much less harsh on the lungs and are probably the ideal candidate for blowing smoke rings.

Variety Is The Spice Of Life!
The velocity at which you exhale controls their speed. Beware that not all ring configurations will be stable, for example larger rings need to move slower to remain stable. Practice making different sizes of rings and you can do some fun tricks, for example blowing a large, slow moving ring then blowing a small fast moving one through the middle of it.