Oh my god man, eat like it's your last supper - gorge on all your favourite goodies that you know will be off limits for months. I had steak and fries the night before mine but it was pizza that I was craving a few weeks in, it's amazing how much you miss texture and crunch in particular.
If you can get your hands on it then coconut water is really great for keeping hydrated plus it's not acidic like juice so better for your teeth. I'm crazy about the stuff generally, nice with vodka too