Overcrowded flights mean airlines will ask for volunteers to stay behind in exchange for a voucher and a seat on a later flight. And if no one volunteers, the last passengers to show up at the gate often get the boot.

You're less likely to lose your seat if you arrive on time and you've checked in via the Internet, paid full price for your ticket or racked up frequent-flier miles. Or fly JetBlue, which fired its CEO to make amends; it doesn't overbook flights and rarely bumps passengers. Avoid Continental, which has the worst record among large airlines.