i dont know if anyone has said this already, but from what ive read about this subject (and i've read alot. mostly on theese forums) the most useful and most reliable information that i've gathered is that holding the smoke in your lungs does no good becuase the thc gets absorbed instantly but not all gets absorbed. in order to get the rest of the thc in that hit into your system, you should exhale and then breathe back in before the smoke is completely out of your mouth. that will re-mix the thc around in the smoke, giving another, (but less potent im sure) hit to you.
i do that usually 2-3 times and i realy do notice it helps conserve. i rarely smoke more than one bowl when i do that method.