The number one rule in marathoning is don't try anything new on race day or race week. Stick to what your body knows and treat every long run like a dress rehearsal for the race.
Remember your training. Keeping to a training schedule and putting in the time is sometimes harder than the race itself. Make sure you train correctly and your body will thank you on race day.
Watch everything you eat before your race. Eat protein and carbs but don't over carb load. You don't want to feel heavy on race day. Promax Protein Bars are a great way to eat your carbs without feeling too full.
Have a mental plan for race day. Keep your mind actively engaged in the race and think of two and three mile increments instead of as a whole.
Don't let the things you can't control affect your day. At the tough times, set a shorter goal to run or walk to an object away from you instead of focusing just on the finish.
Taper your training 3 or 4 weeks out. It can be tempting to add more mileage to your training plan but doing so will only risk you leaving your best miles while training and not in the race.
Review the course. You don't have to memorize the route but studying it prior to the race will help you get a mental plan in order.
Arrive early to the race so you have less stress before you start. Focus on your breath and mental game plan right before the start of the race. The pre-race meditation will calm your nerves, focus your mind, and help you run a strong race.
Avoid starting the race too fast and instead conserve your energy early in the race so you maintain a steady pace throughout the race and finish the race strong.
Don't forget to have fun. All your training has come to this day. Own it, embrace it, and take it all in.
Sources: http://www.active.com/running/Articles/4-Last-Minute-Marathon-Tips-for-First-Timers http://www.active.com/running/articles/13-tips-to-run-a-strong-first-marathon