There is a science to airfare: The day, time of day, and the time of the month that you book matter.
The number one mistake I see most travelers make is to book too early. Unless you are planning travel for high-traffic days, like Christmas or July 4, you stand the best chance for the lowest possible fare 45 days out for domestic travel and 60 days out for international.
Outside of that 45-day window most airline computers aren't programmed to give you any kind of a deal. It's all about computer modeling -- the airline knows, for example, how many people flew on Flight 405 to Cleveland last February, and in February of 2012 as well. They make their projections of the load for this February based on that.
If you book too far in advance, you'll almost always pay a higher fare, and then, if the prices later drop, you can't take advantage of the lower price without incurring the standard change fee -- which will easily erase any possible savings.
When I am in the appropriate booking window for my travel dates, I aim to choose off-peak days, especially over the holiday season. Flying midweek is often an effortless way to save money, and that's when you'll often find emptier and cheaper seats.
Now that I've narrowed down when I want to travel, I follow a few time-tested rules about when to book. There is a specific time if you want to get inexpensive flights.
Buy your ticket on Wednesday at 1 a.m., just one hour after Tuesday midnight. But remember, that's midnight in the time zone where the airline is based (know when to call for all U.S. airlines). Why Tuesday? Most low airfares seem to appear between Sunday night and Monday night. And then, when people book those fares, they have 24 hours in which to purchase them. At midnight Tuesday, all the discount fares that weren't purchased come flooding back into the airline's computer systems. And that's when you strike.
You can't do this online... You need to have a conversation with a human being at the airline. It's also how you book the best economy seats without paying extra.
Some other fare rules I try to follow:
If possible, I aim to book flights after the 7th of every month, as booking is busier right after payday on the first and 15th.
Whenever possible, I shop for fares on one computer and then clear my cookies or book on another computer altogether. If you want to take it a step further, go to a different location. While there's no proof or hard evidence, there is growing anecdotal evidence that suggests airlines are tracking your behavior online, and potentially even tracking IP addresses, and prices could go up for flights that are searched frequently.