From Christine Luff - About.com

If you're new to running in road races, you may not be familiar with some of the rules – both stated and unwritten. While many veteran runners love to see new runners joining the sport, they have pet pevees about inconsiderate behavior from other runners, especially beginners. To avoid annoying fellow runners (and prevent looking like a newbie), be sure to follow these etiquette guidelines when participating in races:

Pay for Your Spot
Running in a race you haven't entered, also called "banditting", is not fair to race organizers, volunteers, and especially the people who have paid to participate.

Line Up Properly
Nothing is more annoying to a runner at the start of a race than having to weave around slower runners after the gun goes off. Faster runners should line up at the front of the starting line, slower runners and walkers at the back. Some races have corrals based on estimated pace or post pace signs. If not, ask runners nearby their anticipated pace, and if it’s faster than yours, move further back. Most races use timing chips, so the time it takes you to reach the starting line won’t count in your final net time.

Don't Jingle
Don't carry loose change or a set of keys in your pocket. They'll annoy those who are running near you.

Don't Take Up the Whole Road
If you're running with a group, try not to run more than two abreast, so others can pass you.

Show Appreciation to Volunteers
Say "Thank You!" to race volunteers who hand you water or put your medal around your neck. They're volunteering their time and the race would not be successful without them.

Thank Supporters, Too
Acknowledge supporters who cheer for you as you pass them. If you're too tired to say "thanks," show them a smile, wave, or give them a thumbs up. It will make them feel good and encourage them to keep rooting for others.

Be Careful at Water Stations
Water stations can get a bit chaotic and crowded. Use caution when running into a water stop and make sure you're not cutting off other runners or spilling water on them. If you're going to stop or slow down to walk through the water stop, make sure there's not a runner behind you.

Keep Moving at the Finish
Don't immediately stop at the finish line or in the chute. There will be runners coming in right behind you, so keep going until it’s safe to come to a stop.

Don't Be a Glutton
Don't take more than your fair share of food and drinks at the finish line. The back of the pack runners will appreciate it when there are still enough goodies for them at the end.