I completely agree with drinking as much water as possible. If you are worried about having to go to the bathroom or cramps, eating sliced cucumber or watermelon can help keep you hydrated. They are also easy and light snacks that can help settle your stomach if you have a hard time eating before a competition. Also, be sure to drink as much water as you can the day before your competition. Have a couple extra glasses of water before you go to bed. Staying hydrated on the day of a competition won't help much if you are dehydrated from the day before.
Eating healthy the day before a competition is very important, especially if you find it difficult to eat before you compete. Avoid fast food and high-fat, high-sugar meals. Whatever you put in your body the day before a competition will play a large role in your energy level.
Also, pay very close attention to your breathing when you are warming up. This might sound a little silly, but make sure you are taking regular breaths. If you find yourself getting out of breath easily, you may be holding your breath. It will help to work on your breathing for competitions during practice, even if you don't find you have this problem while practicing. My CP had this problem when she started flying. Her coach had her work on breathing in and then out at certain points in her stunts during practice (high-V: breathe in, cradle-down: breath out). You can do the same if you are a base. After a while, it became habit for her and she no longer comes off the floor completely out of breath.
Finally, cardio, cardio, cardio. I prefer running. Unless you really enjoy running, you don't need to run long distances. Running one mile, three to four days a week while working on increasing your mile pace with each run will greatly increase your stamina.