Flexibility is the key to finding cheap airfare. The time of year, the day of the week, and the time of day all affect the cost of your airplane ticket. Flying at the right time can literally save you hundreds of dollars.
Many destinations have a high season, with elevated visitation and expensive airfare, and a low season, with more moderate visitation and cheaper airfare. When planning your trip, shoot for the shoulder season, or the time period between the two seasons. During the shoulder season, the weather is still typically quite nice and the airline tickets have begun to come down in price. Once you choose the time of year, you can focus on the day of the week. Tuesdays, Wednesdays and Saturdays are generally the cheapest days of the week to fly, and trips without a Saturday night at the destination can sometimes be more expensive. If you really want to cut costs, schedule a red-eye flight. These flights usually take off and land in the wee hours of the morning, but they're typically deeply discounted due to the inconvenient time.
There are some great Web sites to help you find these deals. Sites like Orbitz.com or Travelocity.com allow a customer to enter a departure and return date, then search for fares up to three days before and after each of these dates. Other sites, like Itasoftware.com, allow users to search rates for the entire month.