S-Kill shares his tips that are very simple but most people forget them.
1) Stay hydrated. Drink as much as you can--even more than you think you want. It really helps with the stress and maintaining a good physical balance. I'd suggest resting up, but that can be really hard, while you can always drink some more water. You should eat something, but keep it light. You don't want to be lightheaded, but hungry > full.
2) Practice your core stuff. You basically need your mechanics to be at the place where you don't need to think about them, and they are automatic. If you haven't done that, you create a huge amount of extra stress when you're in the match, and it prevents you from playing the best mind-games you can if your attention is split. Being confident in your mechanics also really helps overall with motivation--I could always mentally deal with losing or being outplayed, but it was devastating to me to know that I could have won, or actually outplayed someone, but I beat myself by not playing to my ability.
3) Stay in the moment. Practice ahead of the tournament, but be focused on your current match--don't think about who you have to face next in the bracket before you've beaten your current opponent. Give them that respect and you won't get caught sleeping.