You've been training for months. You've run the distance, now you're anticipating the big day. The day before race day is still part of your training. It is important not to forget that you've got a big run ahead of you, so your body needs to stay prepared.
The day before. . . . .
Continue to hydrate - Drink plenty of water the day before your race, as well as the entire week leading up to race day. Just because you aren't running as much, doesn't mean your body doesn't need fluids.
Fuel properly - What you eat the day before your race does matter! Continue to eat regular, healthy meals. Skipping meals or downing junk food is not an option. Your body needs proper fuel, especially if you are running a longer race. You don't necessarily need to carb load the night before a race - current research is moving away from this concept - but eat a well-balanced meal that your body is used to. My favorite pre-race meal is spaghetti with meat sauce and salad. The protein, carbs, and veggies work great for my body when running long. Figure out what works best for you and stick with it.
No late-day caffeine - If your body is sensitive to caffeine, don't have that late afternoon cup of coffee. The last thing you want is to miss out on much needed sleep on this all-important night.
Charge your electronics - Do you need your iPod or MP3 player for your run? Charge it! Losing your music half-way through a long run can be a real distraction. Avoid that pitfall by making sure you've got sufficient power. Do you wear a heart rate monitor or GPS device to help you run the correct pace and/or track your mileage? Make sure it's on the charger before you go to bed.
Check the forecast - If you know what elements you'll be facing, you can choose the proper running clothes and gear.
Get your clothing and gear out and ready - Once you know what you're going to wear, get it out. You may even want to try everything on to make sure it's comfortable. Last minute wardrobe changes can eat up your valuable morning prep time. And, while you're at it, get out anything you plan to bring with you; like gels, snacks, water, anti-chafing product, chapstick, gum, money, etc. Most importantly, make sure you have a pocket or somewhere to put your car key or whatever you need with you on the run.
Check the race website and/or informational email - Double check to make sure you know what time the race starts and where it is. If you haven't been there before, map it out in advance so you know how much time you'll need. Getting lost on the way to a race is definitely no way to start your day.
Set your alarm - Need I say more?
Go to bed at a decent hour - Get a good 8 hours of sleep if at all possible. Your body will thank you in the morning and throughout the race.
You are ready. Don't forget to enjoy the run!