It may be tough to hear, but it's important: Register to receive newsletters and weekly deals, and not just from one airline.
Many of us have sworn off such things entirely, having had our e-mail address sold to spammers one too many times. But if you really want the lowest possible fare, this sign-up borders on critical.
Airlines want you to want them, and only them, and in exchange for your expression of even mild interest they'll send you, free of charge, ridiculous promos like 20 percent off every flight and buy-one/get-one tickets. And you'll fairly regularly see more modest deals like $30 off your trip, or temporary, seasonal price reductions that you'll only hear about if you're on their mailing lists [source: Airfare Watchdog].
If you want to limit your registrations, just sign up for the airlines with which you have a frequent-flier account, since you probably end up flying those more often anyway.
And speaking of those miles: If you're booking a flight from, say, Philly to Chicago, don't use them unless you have to. They can be put to better use toward something that gets you across the country or abroad [source: Hobica].
And finally, an imperfect, sometimes misleading policy that gets lots of flack but still can be extremely beneficial under the certain circumstances.