I like to book as soon as they come out. We only fly JetBlue. If the fare drops, I call and get a credit. For our upcoming trip (Feb 09), I booked on Sept. 4th. I had a credit from our previous trip, due to lowered fares, for $120!
Some people like to wait & watch fares. I guess it's a personal choice. Since I am pretty sure we will travel every year, I am okay with paying "whatever" and then watching my flights for fare drops so I can get credits for the next trip.
FYI, you might have better luck with this question on the transportation board.