If at all possible, book your flight with Airtran, Southwest, JetBlue, Delta, Spirit, or USA3000. Most airlines are going to hit you with a $100 change fee, but JetBlue only charges $25-$30, Airtran and Delta just $50, Spirit $25, and Southwest nothing at all (Southwest issues a credit good for a future flight). Uniquely, to my knowledge, USA3000 permits a name change for a $35 fee, meaning that you can re-issue a ticket in someone else's name if you decide not to use it. If the fare is your "fare class" has gone up between the time of initial purchase and your new flight date, you will have to pay any difference, however.