Applying for credit cards is the easiest and fastest way to get a boat-load of airline frequent flyer miles. If you want to know the best “how to fly free” secret, applying for credit cards is it.
You can find credit-card offers from all the major airlines: American, Delta / Northwest, Continental / United, Southwest / Airtran, Frontier / Midway, etc.
Generally you just apply for a card, spend some amount of money (often $750 within 3 months), and they will give you enough free airline miles for a round-trip in the United States. Often, this means that you can fly from Miami to Anchorage and back at no (or negligible) cost!
So, credit card offers are the best deal going for how to fly free.