U.S. law requires airlines to let you cancel a reservation within 24 hours or place it on hold for 24 hours before purchase. Check which approach your airline uses, and take advantage of this rule to book a cheap fare as soon as you see it. Waiting to plan your schedule or confer with family often means missing out on good opportunities. Some "mistake fares" last only minutes or hours.
Worried about finding the best price? When you see a cheap fare you can live with, just take it. It is almost impossible to predict when fares will be at their lowest, so settle with what you can abide and live to fight another day.