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 MarathonGuide.com and 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.
How do the crowds in a race slow you down? Courses are measured on the straightest line, cutting the tangents from turn to turn. Every time you deviate from this very direct route between two points, you add distance. In crowded marathons runners have to move around constantly - weaving in and out of other runners. Many of my Ecoach clients have worn GPS and accelerometer technology devices. The amount of extra distance covered is between .75 and 1 mile farther for most. Large races are fun, give you a great tour of most cities, and are supported by crowds that make the experience motivating and interesting. But small races are better venues for running fast.