It’s pretty rare, but occasionally tickets will go down in price. Usually this is because of a fare sale (advertised or UNadvertised) or change to the market that dictates decrease in demand. Southwest Airlines’ entry into the Detroit – Baltimore market a few years back caused prices between the two cities to plummet because supply went way up and demand stayed the same. These days I can get away with a ticket between the two for about a hundred bucks, where earlier it was almost twice that much.
Carriers often have seasonal sales when demand goes down (so book tickets in the high season even earlier!). Three to five months before the winter “low” season starts, you’ll often see advertised fares a little lower than market on some of the major carriers. In my experience, however, these are minor in comparison to some of the larger “unadvertised” sales that will sneak into the fare schedule.
Airlines will also occasionally squabble among one another for demand among a particular city. Last year Delta suddenly dropped fares between Chicago and Buenos Aires (a United hub), prompting a miniature unadvertised fare war between the two carriers. Those in the know jumped all over the tickets and ended up getting 600$ tickets to South America.
The point is that fare sales are fairly few and far between; the primary factor for pricing is still volume of open seats, so its foolish to assume that a great ticket will swoop in and save the day.
One trick that few travelers keep in mind, however, is that most airlines have a reticketing policy on their tickets. That is, if the price of your ticket goes down, you can call, reprice your ticket and they’ll give your extra money back. Often there is a reticketing fee and occasionally they’ll only refund your money in a voucher form, but it’s still currency you can use at a later time. Check with your carrier to see what the specifics are of their reticketing policy.
Knowing this, it’s best to run a search every week or so to see if a miracle has occurred and your ticket price has gone down. If the discrepancy in sale price is great enough, give the airline a call and make some changes to your fare.