Eat a good meal with a 3:1 carb/protein ratio. Avoid new foods and foods that are high in fat content. Believe it or not you want to try to avoid whole wheat pasta, breads, etc. and lots of salad items and veggies 24 hours before the race. The trick is to bring food sources into the body that can be easily transformed into fuel for race day. Too much or the wrong kind of fuel source can produce undesirable results. Try to reduce your intake of coffee, tea and alcohol in the days leading up to the race as well as they tend to dehydrate you. All of these things can be troublesome on race day.
Rest well in the nights leading up to the race. You likely will be anxious the night before race and if you are setting the alarm clock to wake up in the middle of the night your sleep pattern will suffer anyway. Try to get a good night sleep two to three nights before the race while you are able to and there are less distractions/demands on your mind.
Relax the day before. Run a mile or two to release nervous energy. Try to limit activities and deadlines. Don't rush through packet pickup and if at all possible stay off your feet. Set out your race supplies before you go to bed. It's one less thing to worry about before trying to fall asleep and one less thing once you wake up in the morning. Try to eat something light two hours before the race. Water, a bagel, banana, some granola and/or Greek yogurt are great options. Again, nothing new. If you take energy gels, consider taking your first energy gel 10-15 minutes before the start of the race.
Most importantly... don't forget to have fun. You've spent countless hours training for race day. The least you can do is try to enjoy it!
