Most airlines like allegiant, jetblue, etc will post flights 6 months out. So for December you're looking at June before they post flights.
I don't know where you live, but look into AirTran. (go directly to their website for the best rates) They have consistently been the cheapest for us. If you are willing to drive to a larger airport, usually your flights are much cheaper. We live near the Binghamton, NY airport but are flying out of Allentown, PA (2+ hrs away) because the flights were over $200 cheaper (each tkt). It's worth the drive.
Airtran's and Allegiant's flights tend to go up in price the closer you get to the date so if you can book 5-6 months before it's best. Also, fly on a Tues or Wednesday and it will be cheaper.
The other tip is to check the prices of your flight on a Tuesday night or Wednesday morning. Most of the airlines will post cheaper flights then and then raise them again on the weekend when people have time to book vacations.
Hope these ideas help!
Jeanne