You're at the end stretch so work on sleeping and stretching to avoid injury. Use ice baths to recover after longer runs and avoid massages before the big event—but feel free to make an appointment now for a few days after the race (a sports massage one or two days after the event could help speed up recovery).
The night before race day is about fueling and hydrating. Indulge in a guilt-free pasta (bring on the bread basket) dinner, and be sure to drink lots of water. While it's important to sip water consistently the morning of, you don't want to be stopping at the bathroom every other mile and you don't want to run with a sloggishy stomach.
For fall and winter races you'll want to layer up in the morning to keep muscles warm. At most large marathons there will be a Dumpster near the starting line where you can discard extra layers—or simply hand them off to a friend before the race starts.
Above all, have fun and and enjoy the sense of accomplishment that comes with taking on a race. Oh, and wear your medal for the next few days, you've earned it.