Drink sport drinks instead of water the night before your 5K to ensure that you are properly hydrated--sport drinks or fluid replacement drinks contain important electrolytes.
Check the color of your urine--it should be nearly clear. If your urine is not clear, you aren't drinking enough liquids.
Eat your last large meal 12 hours before the race begins. Select grilled chicken or tofu for protein and eat a good-sized portion of regular pasta to provide carbohydrates for energy, with any low-fat sauce and low-fiber vegetables you like.
Pack your sport bag with safety pins (for attaching your race number), a small sport drink, an energy bar, a banana, a small towel, sunglasses, sunblock and any other small, essential items (e.g., asthma inhaler).
Lay out all of your clothing for race day--including socks and undergarments--and pack an extra change of comfortable clothing to carry with you so that you can change after the race.
Prepare your bed ahead of time, so that you have clean sheets and a comfortable place to rest. Lay down half an hour before you need to be asleep (i.e., allow 8.5 hours before your alarm) to allow time to unwind and fall asleep.