I hate to revive a dead post, but there's not many helpful replies here, so I figured I'd post one... after all, this is the thread that brought me to these forums ;-) So here's my new user contribution.

Your lungs work more "efficiently" in the first few seconds of a breath. As you hold the breath in, the efficiency drops. The reason is because your lungs are used for more than gas intake... they're also used for gas release (or gas exchange.) They take in air (or marijuana smoke,) and release carbon dioxide. When you breathe in, your lungs fill with air but ALSO start filling with CO2 that your blood is dumping into your lungs in preparation for release when you breathe out again. The longer you hold your breath, the more CO2 builds in your lungs, and it begins to crowd with the air (or marijuana smoke) making it less likely over time that an air/marijuana particle will collide with your lung wall and be absorbed into the blood stream. For chemists, think solution concentration.

What does this mean?

Tip 1 - Breathe deeply when smoking. Your lungs unfold further as you take in more air, so the deeper you breathe the more surface area you are opening for THC absorption.

Tip 2 - Don't hold in the smoke too long. Ten or so seconds is plenty. Diminishing rewards are not worth it. Instead, exhale slowly HALFWAY!!! Before you get to the end of your exhale, try breathing in again and letting the same hit linger a second time. It won't help much, as you're also keeping more CO2 in your lungs by doing so, but it will help some.

Hope this helps ;-)