I've found having an account and alerts set up on
http://www.yapta.com/ to be invaluable. You just tell it the route and days you want to travel, check off the options that you fancy (direct, one stop, two stops), then it'll update you weekly by email on fare prices and trends (although you can get up-to-the-minute results any time you log in).
I've tended to watch prices trend down, then book when they've gone down a few weeks in a row. They usually bounce back up after a down-trend period, in my experience.