1. Don't drink everything in sight. Those volunteers holding tiny dentist cups of Gatorade seem like angels from above when you are parched and sweaty, but resist temptation. While hydration is important, being bloated is the least fun state to complete a run in. The trick is to drink lots of water the day before and be selective about intake on the day of. It will amaze you how long your body can sustain itself for.
2. Stick to your plan. Run the way you trained -- whether that's eating gels every 45 mins or a banana halfway through, avoid introducing anything new on the day of.