When a flight hasn't sold out at least a week before it's scheduled, the airline will often open up a handful of first- or business-class seats for a bargain-basement number of miles. And even if you've already purchased a reward ticket, you can re-ticket it at the new rate and get a mile refund. "You might have to pay a fee, but you might also be saving thousands of miles," says Brian Kelly of the website thepointsguy.com. Create an alert at expertflyer.com, which will notify you when rewards seats open up on an upcoming flight.