Avoid crowded races. Most runners in the large population events run more than three-quarters of a mile farther than marathon distance. Since you don't get credit for the extra distance, it is harder to run goal pace in these venues.
Look for smaller marathons that have a good record for producing fast times. Visit www.MarathonGuide.com and www.RunnersWorld.com for a lot of information with runner reviews, which are usually quite revealing.
Courses that have slight upgrades of short duration allow for a varied use of muscle groups, which often helps in reducing muscle fatigue. There are many downhill courses that can help speed you up – if you train on downhill courses.
While you cannot control the weather, look for venues that tend to have high temperatures of @ 60F (14C) or cooler during the month when you are running your race.
From Jeff's Galloway's Marathon FAQ