Free airline miles!
Another strategy I use to earn free miles is eating. Yes, eating. Many airlines offer miles for dining programs run by the Rewards Network. All you have to do is to register a credit card with the program, and whenever that card gets used at participating restaurants, you get credited miles. Sure, I don't eat out that much.... but my parents do. With the frequent bonuses they were giving out last year, I earned several thousand free miles through this program. The only complaint I have about this program is that the number of restaurants is kind of limited,
but beggars can't be choosers, right?
If you want to learn about some more ways to earn frequent flyer miles, here are some other resources, for your edification:
Flyertalk - Sort of the FatWallet of flying. Members post deals, experiences, and anything else you want to know about getting the most air for your buck.
Free Frequent Flyer Miles - I just found this as I was writing this up, but it has some useful information about ways to earn frequent flyer miles. I guess it's kind of the Cliff Notes version of the above. I especially recommend the Student Programs section.