Depending on what you are looking for, I think it might be a bit early to book for December if price is one of your main concerns. We went in early December last year and I got a great airfare in mid-August, but no kidding, I was on-line twice a day, every day checking rates, the very second I saw a good price I was happy with I booked it. The very next morning it went up $75 and the next day it was $109 more! Since I'm always hunting down airfare for someone, I've noticed that it SEEMS like once Southwest announces a sale for the dates you are looking for, the other airlines offer sales or lower prices too. Currently Southwest's sale prices are going on for travel through September 15th.
The best thing to do is to set a "goal price" you will be happy with and as soon as you see that price or something close to it, grab it!