For our January 2012 trip, we purchased airfare in the first week of August 2011.
For August 2012, we purchased in mid January.
For December 2012, we purchased in early July.
For August 2013, we purchased in early January.
With each of the 2012 trips, prices only increased from the time of purchase (in most cases, VERY substantially) in all cases except for one (our sole Airtran flight). None of the others, all with Jetblue, went down. And believe me, I looked daily. Or even multiple times a day. In the case of the Airtran flight, they put the $20 pp difference in a travel bank for us that we used to upgrade to business class at the 24 hour mark.
Our August 2013 flights have already increased since purchasing a little over a month ago. Considering the amount of time between now and our travel date, the plane is surprisingly full already
.
If we had waited until six weeks out to book our flights for ANY of those 2012 trips as one of the PP's have suggested, we would have been paying a bundle for those tickets.
I also like booking early to ensure we get the flights we want, the seating configuration we want (we all like the aisles.....DS and I across from each other, and DH behind DS), and the location within the plane we want.