(All this pertains to domestic air travel.)
There are many schools-of-thought. At the minimum, you cannot make airline reservations before (approximately) February 20, 2007. At that time, many (but not all) airlines release their TENTATIVE flight schedule for the return date of your trip. Keep in mind that this schedule could change over the course of the year, and you could get bumped a few hours ahead or behind for each flight. You can try to get bumped back to a more amenable flight, but that's not guaranteed: You may just end up with a refund and having to start all over again. Flight schedule chanages tend to calm down around five months prior to the flight. Not coincidentally, that's when the airlines that don't announce flight schedules eleven months prior announce their flight schedules.
People disagree when, along this eleven month period, is the best time to buy airline tickets. I think it's pretty random. Sometimes the best deals are right at eleven months. Sometimes at five months. Sometimes later than that. Sometimes somewhere in-between. Generally, with the deepest discounted rates, there is a penalty for changes, including fare changes, so after you purchase your tickets, airfares would have to go down a good bit before it would be worth trying to switch to the lower airfare.
Next you need to determine where to purchase your tickets. Some folks like to buy their airline tickets from where they're buying their vacation package. That's a good thing when the package itself is for something that moves -- something that if you're late you "miss it" -- i.e., a cruise or a bus/train tour. Otherwise, I don't see a benefit to that, as you typically have less choice about which airline, which flight, etc., and less overall control over your reservation. Some folks like to go through a
travel agent, but again, I feel that doesn't add much and provides less overall control over your reservation. I like to buy directly from the airline, using a search engine like sidestep.com or kayak.com to search for the best airfares.
I hope this helps.