Don't start off fast.
It's very easy to let the excitement at the start of the marathon get to you and before you know it, you're running way too fast. The first few miles are crucial to having a good race. Decide well before the start what pace you want to begin at and and stick to it. It might be hard to see lots of runners flying past you at the start, but trust me, you will catch them all in the second half because they broke rule #1.
Don't change your race plan mid-race.
Do not stray from the your pre-race race plan. You're only asking for trouble if you do. If you're feeling good early on and you think you could run a little faster than you planned, that's great. But, instead of picking up the pace, maintain that feel good pace you're running and conserve your energy for the push at the end. You'll be glad to have that little extra in the tank.
Don't try something new for the first time during the marathon.
Race day is not the day to try that new running shoe, sock, short, shirt etc... Same goes for in-race nutrition. Don't eat or drink something you've never tried before. On race day, stick to what you know works. You don't want to find out halfway through the marathon that your running shirt is rubbing you the wrong way or the energy gel you tried made your stomach upset. Training runs are the time try out new things, not a race.
Run happy and enjoy the 26.2 mile journey!