a few things that work for me:
1. ephedra/ephedrine. you can either buy the ground up herb (and make into tea) at a health food store or get it in pill form. i try to go easy so i don't become hooked, usually take 2-3 during a high. helps stay focused and energized. i get nasty burnouts too, especially now that my tolerance seems pretty high.
2. i also find certain teas to be quite helpful with keeping you buzzing. my old roommate once made a mysterious tea concoction with a good amount of cinnamon, and it was amazing. totally changed my high and gave me lots of energy. and as the other dude said, i'm sure green tea helps. experiment.
3. this can be hard when you're stoned and lazy and it's -10 outside.. but oxygen. oxygen and fresh air definitely wakes you up
4. change of pace. sometimes if i've been doing something for a long time (listening to music, doing art, playing video games, whatever) just changing my activity is a good refresher
5. just remembered another: peppermint is a great natural stimulant. i'm not sure how powerful the tea is for combating burnout, but I have a little bottle of peppermint oil which is quite potent. put a drop under your nostrils and breathe in, or burn the oil in a diffuser.
-- question:
do people burnout faster/more intensely when smokin from the bong? i wish i didn't burnout so fast and so hard.. but my tolerance is quite high so it's hard to go back to pipes. doesn't affect me nearly as much. what do you use and how's the burnout?