Now is the time to go for your loyalty program’s mileage-purchase limit. The deadline for all but three programs (see chart below) is midnight, December 30. Buy the limit now and buy it again in January, and you’ll be positioned to score a major upgrade in 2008.

Example: Buy 19,000 British Airways miles now (the yearly limit) and 6,000 in January and you’ll have enough for a Business Class upgrade from the US to London—at a cost of only $737.50! The upgrade is valid with fares as low as $686 round-trip, whereas a Business Class ticket costs $2,900 to $12,000+.

Bonuses: This is also the time of year that the airlines run mileage-purchase promotions. Delta is offering a 20% bonus on each 10,000-mile purchase; Alaska, American, and United a 10% bonus on any mileage purchase. This brings down the cost of a mile from 2.75¢ to about 2.2¢ for Delta. For American and United, 10% through the end of the year—bringing the cost down from 2.75¢/mile to about 2.5¢/mile.

Here's where to stock up on miles ...