Basically, the advice that everybody is giving is more of a "how to not ruin your high" rather than "how to extend your high". I think the only way to stay high longer is to ingest the weed in food, as some people have already said.

The best way to keep your high going is to smoke more obviously, but if you're worried about keeping your high going, it probably means you don't have any more to smoke. The secret is to find a way to get high off a small amount of weed, so that you conserve your weed, thus extending your high because you have more weed for later, AND it keeps your tolerance down.

Gravity bongs work great for a small amount of weed. Knife hits supposedly work really good as well, but I've never tried it.

Another thing that me and my friends have experimented with (and came to the conclusion that it works) is to dip your roach when you're finished with a blunt or joint! You can do it as soon as you finish, or you can let it sit for a little while and dip it later (but by then you'll be more likely to just smoke it). Just put it behind your lower lip, and eventually you'll begin to feel a slight burning sensation. I usually keep mine in for as long as possible (usually about 30 mins b/c I end up needing a cigarette), and it definitely seems to make my high better.