Back eons ago when I first started traveling there was a fantastic benefit to booking early. I mean seriously early. I would book my trans-Atlantic or trans-Pacific flights sometimes a year in advance and then would recheck the prices over the course of the next year to discover that I had save sometimes as much as 50% by using my strategy.
As the years have rolled on this is not really as beneficial as it used to be because I have noticed that the airlines, probably due to technology, don't really need to get the plane filled up that early and by offering financial incentives to do so, they just end up losing money because of it.
There also used to be this trick that really only applied to the United States where it was cheaper to book a round-trip ticket and just not turn up for one end of it than it was to book a 1-way ticket. This was just a ploy that was taking advantage of business travelers and doesn't really exist anymore. These day, round trip flights generally speaking are the same cost as 2, 1-way tickets to/from the same destination.
Over the years the various formulas that airlines use to determine how much they can get outta ya for travel has evolved and in many ways the plethora of online booking agencies contributed to this.
I personally feel as though it is beneficial to book directly with a particular airline rather than using a 3rd party like flights.com or whoever the hell does this anymore, but different people have different experiences.
So what about getting the best rates just as a general rule? Well there has been a ton of research done on this and a lot of the results could simply be a coincidence but it has recently been determined that the "sweet spot" for securing lower fares isn't a year ahead of time, it is 38 days.
Why 38 days? I have no idea but I have some thoughts as to why that might be. I think it is because at about this time, around a month before the flight, especially if it involves some sort of super-gigantic airplane that is going all the way over the pacific, this just over a month time before go time is when the airlines start to panic about not selling enough seats and therefore will start to lower prices. This lowering of prices results in the airline selling enough of a percentage of the entire plane that they will shortly after this "sale" jump the prices back up because now they no longer need to entice people to purchase based on price. IF they make additional sales, fine but at that point perhaps they are already in the green and any additional sales is just a bonus.
One time of year that you should definitely book well ahead of time no matter what is the Christmas / New year season for any western country and also various other holidays such as Chinese New Year / TET in Asian countries. These dates see the rate jet jacked sky high, especially if you are attempting to book last minute. Why do airlines do this? Because like most things in business it is a question of supply and demand and they will charge as much as they feel they can get away with.
Why 38 days
This is just a number that was determined by looking at thousands of flights and the fluctuations of prices over a multiple year period of time.
Of course the prices can vary depending on how much flexibility you need but in my case I am able to get quite low fares because I am absolutely certain that I am going to travel on the dates that I determine beforehand.
I have been burned a couple of times because of this but that is the nature of the beast that is low-cost travel.
So the next time you are trying to book a flight ahead of time, play around with the 38 day time period and see if it works out for you. Statistically speaking, it has worked for a majority of all flights internationally for the past year or so.