I do. But you have to tell me what the winning lottery numbers are.
Seriously, the best answer is "when you can afford them". No matter what you may read, there is no GUARANTEED time to get the lowest (I'm guessing that's what you mean by 'best') price.
Some have had luck booking as soon as tickets are released, some have had luck waiting until a couple weeks out.
The best advice is book when you can afford the price. If you are on a legacy airline (Delta, United, USAir, etc), and it will bother you if you find out the prices drop after you book, don't look after you have your tickets. If you're on SW or JB, you can get credit if the price drops, but there are limits. On SW, only the person who's name is on the ticket can use the ticket, AND it must be used within one year from PURCHASE, not scheduled flight.