In my experiance stay away from the stuff. I used to down a can before I got in the ring and I found that after the first round you start to crash, also I found that it really effected my balance and because im a kickboxer this was not good. I also used to drink a can before my weights sessions too but I started to get alot of chest pain and heart palpations so I put it down to the high caffiene in the Red Bull and as soon as I stopped drinking the stuff the pain and palpatations disapeared. Now I only drink Powerade but I find the best thing before I get in the ring or do a weights session is eat a Bannana, it works so much better for energy and lasts longer.Also like the others posted listen to some music that gets you buzzing before a fight!