Only pack a bowl that is big enough to clear in one lungful. Even if you plan on doing more than one hit, repack the bowl every time.
DO NOT USE JOINTS/BLUNTS!!!!! I can't stress this enough for efficient smoking. The ember will continue to burn even when you aren't taking a drag which is smoke that won't reach your lungs. Also there is smoke that comes off of it while you are taking a drag too. So more waste there.
The first smoke of the day will ALWAYS feel the best unless you take a much larger dose later on, so make that one dose count!
If possible, you may also look into getting some mediocre bud for your regular smoking pleasure and only use your good potent stuff for a very special treat.
Also, be greedy. Friends are wasteful if you share
This one of course you'll have to decide on for yourself since smoking with friends can be fun. I just hate watching them waste the marijuana that I paid for (which they do a lot since they don't care. It's not theirs after all
)
EDIT:
I almost forgot about this. I do it too, but i use a plastic cap that fits snuggly over the bowl without touching the ember. any excess smoke gets trapped in the cap which can be sucked out after the cherry stops burning