Here is a post I wrote in 2005 concerning the concealment of smoke and/or smell during a session. I still consider it somewhat definitive and mostly comprehensive.
Quote Originally Posted by lateralus
You aren't necessarily bound to get caught. You are, however, if you choose to use the wrong utensil. Joints, blunts, spliffs, or anything that continually burns is a no-no. Double-O stoners are better off using either a bong, pipe, one-hitter, sneak-a-toke, or anything that can be *immediately* put out after the hit is taken.

Optional: The next precaution I take is to light a single stick of incense five to ten minutes before the session. I use Nag Champa brand incense; it has a great aroma, and a decent potency. By far the best I've found. Of course, if this would raise suspicion among the enemy (YOUR PARENTS), then this step should be avoided. Many people believe that parents know what's going on if you use incense; mine, however, do not. I tend to avoid using the towel under the door because if someone was to barge in, they'd take one look at the towel and know exactly what it was for. Plus, if the following methods are done correctly, the only smell escaping your room will be that of Nag Champa.

After lighting your piece, always put your finger, a marble, or a coin over the bowl to prevent excess smoke from escaping the bowl, and to save your weed. Overlooking this is, what I imagine, what gets many people caught.

As for the spoof, ("dryer sheets thingy") I use a 20 oz Mountain Dew bottle stuffed with somewhere around thirty or forty dryer sheets. The right amount of dryer sheets is key: too little and the covering of the smell is insufficient, too many and you won't be able to blow the smoke through. Some people add an amount of cologne, or spray to the sheets for better results. In my experience, this was unecessary, but better safe than sorry. Holes were burnt in the bottom of the bottle (as recommended by an ancient mello as hello post) using the cherry (lit end) on a Camel. The smoke is blown through the mouthpiece.

Don't try to hold in the hit for too long; if you cough, your whole operation's purpose has been eliminated. I hold mine in for three to four seconds and blow it quickly through the spoof.

I have a stash of Icebreakers (mints) for when each session is over. I also keep an extra can of Axe (a lot of people use Febreeze or Ozium) close to me in case of a coughing fit, or if smoke somehow escaped the piece.

Mission accomplished.
This method is almost fool proof. You'd be surprised at the results.

A related and also potentially dangerous situation is when you've been lucky enough to arrive at the possession of dank that you can smell 4,149 yards away. Weed that will saturate your room, hallway, and bathroom with marijuana odor for hours after only opening the bag for a few moments. I recommend someone with experience in this area comment with suggestions.