The "cheap" seats seem to sell out pretty quickly. Sometimes the airlines will run sales, especially internet sales for certain dates.
What airline are you using? If you are using Southwest book it, when the rates go down you can get the lower rate and have a credit for a future flight.
My experience with airtran is that if you book with them and the rates go down you are out of luck...and they run periodic internet sales.
Other airlines will let you re-book for the lower rate, but charge you a fee to do so and then you really are not saving very much.
What I do, if the flight times that I really want are available and still within my
budget I just bite the bullet and book.
If you want to wait a bit, sign up for fare watcher on travelocity, and sign up for e-mails from the airlines you want to book. When they have an internet sale you get an e-mail. With travel watcher you indicate what amount you want to pay and they will e-mail you when the tickets are that price....however you may not get the flight times you want.
Also, can you change your dates? Sometimes flying out or coming back on a different day can lower your price, you have to play around on the airlines websites a bit.