Join Airline Loyalty Programs
For really frequent travelers, upgrades—not miles—are the primary loyalty benefit. Once you attain one of the higher levels—gold, platinum, or whatever—you can almost count on flying in first class with the purchase of a coach ticket.
If you're at a lower frequent-flyer level, your chances of getting a free upgrade aren't so rosy. You may luck out once in a while, but the higher-level frequent flyers get upgraded first, and there are more of them every day.