Most travel-booking Web sites let you to set up alerts to track airfare automatically. These come in a number of different forms, but all of them basically mean the Web site does the searching for you.
Some sites let you set alerts for specific routes (say, New York to Los Angeles). Others are set up to let you track specific flights. The approach may be to set a particular price you want to pay, and when the route or flight drops to (or below) that number, you receive an e-mail; you might also arrange to be notified when the price of a route or flight decreases at all (from the time you set up the alert).
Because fares change so often, it's important to book quickly when an alert arrives if you like the price. If you wait, the fare could be a lot higher by the time you click through.
Another thing to keep in mind: There are factors that almost invariably (and unchangingly) affect the price of any route you want to travel.