Honestly the best method I have come across found is the one described in this video,
, in which the user goes through short cycles of breathing out small fractions of thc before re-inhaling air, slowly titrating vapor out of the lungs in exchange for oxygen. People often hold smoke in because they think it allows more thc to diffuse into the bloodstream. To a degree, they're right but what they don't consider is that the rate of absorption is hardly a constant, because the lungs' walls aren't just one big single membrane that continuously allows air to leech in the blood. That task in preformed by the millions of alveoli that line the walls of the lungs. When you breathe in, these alveolar sacs stretch out like a balloon pulling oxygen into its hollow cavity, allowing air to diffuse into the bloodstream. Then when you exhale, the sacs then expel carbon dioxide by collapsing, ready to be inflated again when the next batch of O2 comes through. The thing is that when a user holds his breath, these sacs stay inflated/bloated the entire time, unable to collapse and re-inflate in order to suck more air into itself, so the only way new thc molecules are getting into the bloodstream is if they happen to bounce into the sac's cavity (which still happens but at a slower rate than when the alveolar sacs are purposely "sucking in" air during diaphragm contraction). By taking short cycles of exhaling and inhaling, you accomplish three things. First, you allow new air to refill the empty space left behind by the exhaled vapor, allowing new O2 to leach into the bloodstream alongside thc. Second, by exhaling you allow the alveoli sacs to expel carbon dioxide from the body, which is their second most important function in concerns to your circulatory health. Thirdly, it causes the alveoli sacs to deflate, allowing their hollow cavities to re-expand upon inhaling thus forcefully sucking in more oxygen and thc molecules. Its also a little known fact that the protective fluid lining an alveolar sac temporary decreases in surface tension immediately following inhalation, possibly allowing easier diffusion between the gases in the lungs (such as O2 and thc) and the bloodstream.