Before a big important tournament you have to do the following things:
Have a sleep. You do not need to sleep and wake up before the very beginning of the game, but you should have a good rest before a long game at a later time of the day.
Thoroughly eat for an hour and a half before the game. Assimilation of food takes time, so try to get your blood started to feed the brain in time for the beginning of the game.
Go to the toilet. It's funny, but you have to be ready for the game and not be distracted by trifles.
If possible - breathe fresh air, make an easy workout, but do not get too carried away. You should not get tired and excited. Just need to saturate your lungs with oxygen.
During the game - be sure to use all five-minute pauses at the end of each hour to move a little, go to the toilet (yes, I already mentioned this, but you do not want to miss the distribution in the middle of the game?).
Hey, do not waste these five minutes to read the fresh mail or look at the pictures with the cats. Do as I wrote above. Slightly open your muscles. Give a rest to your eyes.
Do not abuse coffee and strong tea. Completely eliminate any alcohol. Better fruit (yes, apples is a wonderful, I approve).
If the tournament goes really long and you are hungry despite apples - eat something quickly absorbed. It will be great if there is someone who will help you with the preparation of food - 5 minutes may not be enough. And do you remember that you need to warm up and go somewhere?