Here's a list of many marathon tips I have shared with athletes. I will be elaborating on them during 26 days of running tips leading up to the NYC marathon this year!

Don't overeat the day before. A big meal is a myth. Have simple food & avoid too much dairy and fiber.
Stick to your nutrition plan. Water + Power Gel (more salt) / 4-5 miles. Use caffeine strategically.
Use parallel lacing on shoes to reduce pressure & prevent stress fractures.
Get your body and mind ready with a few "strides" or 100 meter pickups before the race.
Have a phrase like "Easy, Light, Smooth" or "Smile :)" to recall each mile.
Avoid long, loopy strides to prevent fatigue & injury. Improve your cadence to at least 180 steps/min.
Pain is inevitable. Run to overcome pain & replace it with something wonderful. Suffering is optional.
Start slow & finish strong. Easy to say but difficult to do!
Exerting yourself to the fullest within individual limits: essence of running & metaphor for life.
Keep moving after the race. Continue to hydrate and make sure to get some protein within 30min of finishing.
Make running a habit- have a "reward" in mind (chocolate milkshake) and isolate the "cue" (time of day).
Run a race in costume. For the fans & the challenge. A reminder not to take things too seriously.
Rest is a good thing. Running is high-impact, know when to let your body recover, esp. after a big race.
The only way to run faster is to train faster. Find a track & do 800s, w/ 400 easy jog after each.
Run YOUR pace. Don't chase the person who passes you if it's not part of your plan.