You want to wait a few days after the airline first posts their fares, because it will be the most expensive ones first. The reasonable fares will come a few days later.
Try to book your airfare on Tuesday/Wednesday, as early or late in the day as you possibly can. You will see the lowest rates. By the weekend, the fares go back up again.
My theory is that people are usually home on the weekends and discussing their travel plans, then check fares and they are up slightly.
Since I travel alone, I book mine the minute I get into work on a Tuesday or Wednesday around 6:00 am, and it is the lowest all week. Seriously, how many people are online that early to book airfare?
Give it a try.