One slightly 'outside-of-the-box' technique Ive found for making highs last longer, DONT succumb to the munchies. Everytime I get high and eat a load of crap it seems to wear off slightly, but if I try to ignore the craving for crap the high sticks around longer.
Dunno if anyone else does that, but it's something Ive always lived by, plus it means I don't pile on the pounds like I would if I ate everytime I got the munchies.