I don't know if this works on all airlines, but on United, you can check back on the website and see if your itinerary's price has changed.
I'm going to England in August, starting with a group to attend a music festival, followed by ten days of independent travel. I booked my plane ticket last month, and following up on a hint on the FlyerTalk discussion board (
http://www.flyertalk.com), I checked my itinerary and found that the price had gone DOWN $79.60. I was able to get the new price, and I'll receive a voucher for the difference, applicable to another flight in the future.
I also tried repricing my Japan flight, just in case, but it's already a sale fare, and the website told me that I already had the best available deal.