In addition to prices fluctuating as a function of availability, they will also change as the date of travel approaches. Typically, barring a fare sale, tickets are least expensive several months before travel. As it gets closer to the day of travel and demand tightens across the board, airlines will often increase the prices. This usually happens one or two weeks out, although around the holiday season and for international fare this will happen up to a couple of months away.
Also consider if any special events are happening around your destination at the time of travel. If one of the pricing jockeys at the airline know that demand should be higher during that time, they’ll make sure that the ticket prices stay high.
In some rare instances, last minute fares will become available at super discounted rates. Sites like lastminute.com capitalize on buying blocks of last minute tickets, packaging them with hotel or car rentals and selling a bundled deal. With recent volumes and demand as high as they have been lately, I wouldn’t plan on making getting a great deal this way; if you’ve got an important date where you can’t miss a flight, please don’t rely on this method.
The few times that a last minute fare has worked for me have been when I didn’t really need to go anywhere. Sitting at my cubicle on a Wednesday afternoon I’ve gotten great deals from Lastminute and assorted airline sale pages to head out for the weekend. But it was totally spontaneous travel.