Discounted tickets can be found as soon as the booking window opens up for the travel dates in question. 334 days in advance of the return date, if I recall correctly.
I check immediately when the window of time comes up, assuming I'm positive of my travel dates. This year, that really paid off. I got 2 excellent rates on sets of tickets to Fla. in 2003. In one case, the phenomenal price disappeared within hours.
Check websites as often as you can, because discounted fares come and go. Sometimes good fares are totally unpublicized. If you're not already registered on travelocity, that would be a good thing to do. You can sign up for Fare Watcher, and they will notify you by e-mail when rates between your key cities are below a certain price goal that you set.
I also like expedia, orbitz, and cheaptickets. But whenever I find a good price on one of those sites, I immediately go to the specific airline's website to make sure I'm getting the lowest fare for that flight.
Hope this helps a little. Good luck as you look for nice cheap airfare!
