Avoid fare-change fees
The other major revenue source for the airlines is the fee to change a ticket. On most of the larger airlines you'll pay up to $150 to change a reservation, on top of the fare difference. Southwest — by far the most fee-free airline — charges only the fare difference. (And as its ads proclaim, the carrier doesn't charge to check a suitcase, either.) If you anticipate a date change, look into buying a flexible, full-fare ticket rather than paying the fees. If you need to return early, it may also be cheaper to abandon the second half of your old ticket and buy a one-way fare home.
And by no means should you pay stiff penalties or fare differences if you simply want to change your flight to a different time that same day. Most airlines let you do that for a reasonable fee (about $50).
See Also: 3 reasons airfares can go up.