Moreover, people who book at the last minute are usually flying on business, and therefore the business is paying for it.  People flying for pleasure usually know weeks or months in advance, and can't afford the prices that a business would pay.  Thus it is to an airline's advantage to set rates according to the major differences between business and leisure travelers:

• Business travelers fly mostly between 9 and 5, whereas leisure travelers can fly offpeak hours. 
• Business travelers buy tickets on very short notice, whereas leisure travelers plan trips well in advance.
• Business travelers do not stay over a weekend (= Saturday night), whereas leisure travelers do. 

For example, US domestic excursion fares require that you buy your roundtrip ticket 7 days, 14 days, or 30 days in advance, and that you stay over a weekend (usually Saturday night, though sometimes Sunday night as well).  Some may also restrict the travel to a 30-day maximum stay.  Stopovers aren't allowed, except for the purpose of connection (if you want a stopover, you'll have to pay extra). Some fares may be limited to a particular routing (e.g., routes with connections cost less than nonstop flights). 

You may also be limited to flying during offpeak hours, with flights during the busiest times of the day costing more.  Tickets are usually non-refundable and non-transferable, and may either be non-changeable or have a $35 change fee.  The price is usually the average of the two one-way tickets.  (E.g., a 2-week advance PGH/BOS advance ticket is around $200 this way.) Tickets must be purchased within 24 hours of the reservation. 

For international excursion fares, also known as apex fares, you must book at least 21 days in advance of the flight, and you may have to purchase the tickets at the time of the reservation. Minimum stay is usually 7-10 days, and maximum stay can range from 3 months to a year.

Midweek travel (Monday, Tuesday, Wednesday, or Thursday) is usually cheaper.

Tickets are non-refundable and non-transferable.  Fares often depend on the seasons.

Since a regular 1-way ticket is so much worse than a round-trip excursion fare, it sometimes pays to buy a round-trip ticket and throw away the other half (if you're only going one way).  If you buy a round trip ticket and throw away the other half, make the first leg of the trip the destination, since some airlines will cancel the return trip if you don't show up for the first leg. 

If you intend to skip ONE leg of a multi-leg flight, tell this to your reservation agent in advance, so that they can annotated your PNR to prevent subsequent legs from being canceled.  (Note: You can't use this technique to exploit a "hidden city" fare.  You'll have to have a very good reason for skipping a leg for the travel agent to allow it, and you may have to pay a different fare to do so.)

For example, a round-trip to San Francisco from Pittsburgh with a one-night stayover is $1,333.  However, the cost of a Saturday night stayover is only $479 if you order the ticket a week or two in advance.  Purchasing two round trip tickets, one originating from Pgh and one from SF, and then using one half of each round trip ticket saves you $375.

Note that for many airlines the discount fares depend solely on the date of the first leg of the trip.  The price does not vary no matter when the return flight is (so long as you stay over a Saturday night).

If you travel on offpeak hours and low volume days, the rates are cheaper.  Thus to guarantee a low cost flight, you have to be very flexible about where you are going, what time and day you are leaving, and how long you want to stay.  Offpeak hours typically include before 7 am, between 10am and 2 pm, and after 7 pm, depending on the day of the week.

Also important is when you make the reservation.  If you make the reservation for an offpeak flight during the peak season (say, make a reservation for February just before Thanksgiving), you may be charged the peak rates.  After the holidays some airlines lower their discount fares to attract customers.  So you may be able to get a better fare by making your reservation right after the holidays.

If you notice that the fare for your flight has been lowered after you bought the ticket, try calling the airline. Sometimes they will refund the difference between the price you paid and the lower fare.  (You may have to go to the airport to get the ticket rewritten at the lower fare.) You may have to pay a $35 to $50 reticketing fee to get the refund, but some airlines have been known to waive the fee if you get the money back in the form of a voucher (to be applied to future travel) instead of cash.  For example, USAir will refund the difference less a service charge, or give a travel voucher (credit)
for the difference with no service charge.

According to a Wall Street Journal article by James S.  Hirsch, (July 30, 1993) many airlines now test fare increases by raising prices on the weekend (Friday night through Sunday night).  If other airlines don't match the increases, the fares return to normal on Monday.  So you should be careful when purchasing tickets on the weekend.  (This works around the Justice Department consent decree that prevents airlines from signaling proposed fare increases in the computer reservation systems.  When the competitors didn't match the increase, the airlines would cancel the increase before it took effect.  With the new method, since it affects current prices, it technically isn't price-fixing.)

Hirsch also reported that many airlines have increased the $25 charge for changing advanced purchase tickets to $30 or $35.  Note that this is often a minimum fee -- you'll probably have to pay the full difference in price if the new ticket price is much higher.  Airlines are using these measures to discourage passengers from buying tickets during a fare war and later changing the time of flight or destination.  If you go to one of the airline's ticket agents (not a travel agent), some airlines will reissue a ticket at the lower fare without fee if the difference is refunded in the form of a voucher for future travel.

In addition to the Saturday night stay requirement, many airlines now give additional discounts for travel midweek (usually Tuesday and Wednesday, sometimes Thursday) when a Saturday night stay is involved. Some also provide discounts for travel on Saturdays.  Flights usually aren't fully booked on these days.  For example, Northwest discount coupons recently had this restriction.  Most leisure travelers like to travel Sunday-Sunday, or at least not miss a large part of the week. Business travelers, of course, don't like to stay over the weekend.

When making your reservation through an airline ticket agent or through a travel agent, always ask for the lowest possible fare.  Don't just give them specific dates/times and ask them for a low fare -- tell them that your plans are flexible, and you'd like to know what the low fare is.  If you're too specific on the flight details, and don't say that you're looking for the cheapest possible fare, you might not get the best price.  Sometimes by departing on a different day, you can get a much cheaper fare.