Here are some things that I consider when choosing a race:
1. Decide on Distance. Do I want to do a 5k? 10k? More? If I want to do a longer race (10 miles or Half) I make sure I chose one that's far enough away so that I can have adequate time to train. When I did the Black Cat 10 Mile, I gave myself a little over 2 months to train. I knew I wanted to do a half marathon, so I chose one in September so that I would have all summer to train.
2. Find Friends! My first thought is usually: who can I convince to run this race with me? I've only ran one race by myself, and even though I did OK, my nerves were out of control! It's nice to have a friend to run with you, or just to go with you and cheer you on. Before I register for a race, I see if anyone else wants to do it with me. It makes a huge difference in how much I enjoy the race.
3. Research the Race. What does the race benefit? Does the registration money go to a foundation or charity? I'm more likely to sign up for a race if the money is going towards a charity I care about, like the Scott Procopio Gold Star run. This race honors Scott Procopio who was from my home town of Saugus. Scott was on his second deployment as part of Operation Iraqi Freedom, having previously served in Fallujah in 2005. He was killed in action as a result of a roadside bomb on the morning of April 2, 2006 in Ramadi Iraq. There is now a memorial scholarship fund in his name.
4. Pick one with Perks. What do I get out of this race (besides endorphins)? I look to see if you get a t-shirt (or tech shirt), a medal, a beer, food, anything. These are all silly little things, but I want to get the most out of my money, and they serve as great mementos to remember the race.
Again, I'm not saying I'm an expert at races/running/anything, I just want to share what goes in to my personal race selection.