Earlier is usually better when booking flights because the prices increase as the tickets sell - for example, you could book 2 months before the flight and pay, say $500 dollars for the ticket, but the person sat next to you on the plane might have booked 4 months before and only paid $300.
Another tip is to pay for the ticket as soon as you book them (some airlines give you a deadline for paying once booked) because the taxes can change. Back in November we booked return flights from London to Guatemala City for £490 each (including taxes), but didn't pay straight away. By the time we called the airline a few weeks later to pay, the taxes had gone up and we had to pay £580 each instead!
I'll be flying from London to Albuquerque in January and recently found a one-way flight for about £370. However, I'll need to wait until I get my visa in order to book the flight (which won't be until Sept-Oct) and by then I'm sure the flights much more expensive (even now, the prices are ranging anywhere from £360 to £700 on Yahoo travel).