When is the best time to book the cheapest flight? There are websites that try to predict the fares (http://farecast.com). Various travel writers have come up with various lead times for the best deal, frequently either several months or just a month out.
There is another way to get piece of mind, use the tracking service Yapta (http://www.yapta.com). Yapta takes your flight details (either entered manually or by giving it your record locator number) and watches your flight every few hours. If the price goes down, you get an email alert.
It turns out that for most airlines, even if you have a heavily restricted ticket with $150 change fees, you can still get a travel voucher used for future flights if you call and ask.
Not every ticket will trigger an alert; in fact most airfares go up and up. But I’ve had about 4 or 5 go down, resulting in a couple of hundred dollars in vouchers. And it only takes a couple of minutes to enter the information, less if you install Yapta’s web browser add-in.