Yapta.com tracks the price of online tickets. If you bought a ticket for $500 and the airline reduces the fare, Yapta sends you an e-mail letting you know about the fare change. It's totally free.
Then you go to the airline, point out the price drop, and they'll refund you that money. That's the policy at most airlines. All you have to do is send your itinerary to Yapta.com.
According to Yapta.com, the average amount they save for each consumer is $227 per trip.