What you see is rarely what you get when it comes to airline fares. Sure, things got a little better last January when the Department of Transportation mandated that airlines and online travel agencies include all taxes and fees in prices they advertise. But trying to find that $99 fare to Florida you see advertised is often still an exercise in perseverance, if not futility.

Why are air fares like this? Why is it that, no matter what site you’re on, when you see a fare and then do a search you almost never get the same fare that’s being touted?

The problem is that airlines offer a lot of different fares for the same routes. Delta, for instance, currently offers 48 different fares from New York to Atlanta, ranging from $198 to $2,526 round trip (before tax)! The fare that applies for any particular trip depends on when you book your ticket, what day you fly on, and how full the flights are. There are a lot of reasons why there are so many different fares.  But the bottom line is that the airlines employ very complicated “yield management” strategies to make sure that their planes don’t fly with too many empty seats, but that they also don’t sell tickets for one price when someone else would have been willing to pay more.

Translation: less popular flights on less popular days tend to be cheaper. It’s simple supply and demand.

In the New York to Atlanta example, to get the $198 fare you have to book at least 21 days in advance; travel between November 12th and November 20th; and fly on Tuesdays and Wednesdays. There are only 4 travel dates that actually meet those conditions but even on those dates, you have to select flights that are pretty wide open. If you can’t meet those restrictions, other fares exist for $238, $274, $276, $304, $314, etc. all the way up to $2,526 (a fare which likely no one actually pays).

With all these different fares, it’s important to understand that when airlines offer sale fares, they will only set aside a certain number of seats per flight to be sold at that low fare. Once those seats are gone, the next highest fare will apply. That fare, too, will have a limit, so on a heavily booked flight you might end up having to pay, not the lowest possible fare, but the third lowest, or the fourth lowest, or—in extreme cases, the twentieth or thirtieth lowest.

So a lot has to go right to be able to get a fare you see advertised. The dates you want to fly have to qualify in both directions and there has to still be seats left at that fare both ways. Sometimes this is easy, sometimes it is not. Some sale fares are available on 30, 60, or even more dates; others, like the example above, may be available on only a handful. Most advertisements are completely legitimate in the sense that they are available on some days, but you may have to be very flexible to take advantage.

Trying to get an absolute rock bottom airfare is an imperfect science, but here are some tips to help you fare a little better:

– If you’re looking for the best deal, plan your trip to fly on less popular days. Tuesdays and Wednesdays are almost always less expensive than Fridays and Sundays. Other days may be somewhere in the middle and choosing a cheaper travel day in one direction is still better than none.

– Sign up for sale alerts with your favorite airline or travel site; most of them will email you when fares go on sale. Don’t get too caught up in the exact amount being advertised because it may not apply to your itinerary if you’re traveling on the wrong dates. But like a rising tide that lifts all boats, fare sales tend to bring discounts to all days, even the days that don’t qualify for the absolute lowest fare being advertised.

– Unless you’re traveling for a holiday or peak travel time, don’t feel you need to book months and months in advance. For domestic flights, four to twelve weeks before travel is normally the sweet spot.

– Check fares often to wait for a price drop, but when you see a good fare, be ready to pounce.  Many sales are only 24 hours long and seats sometimes sell out even sooner.