Flying standby used to be one of the cheapest ways to travel. The idea
was that an empty seat doesn't earn the airline any money. So some
airlines would sell special fare "standby tickets".  Using such a
ticket you were NOT guarranteed a seat on a particular flight, but on
the next flight with empty seats. You'd be enplaned on a flight
subject to the availability of space at departure time.  This was only
after all passengers with reservations for the flight have been
boarded.  Passengers from a previous flight who were bumped also had
priority. No stopovers were permitted on standby fares.

Since advance reservations were not accepted, you had to get to the
gate early to put yourself down on the standby list.  If it was a busy
day and the flights are full, you would have to wait several hours to
get a seat, or maybe not get a seat at all.  A standby ticket didn't
guarrantee you a seat, but if you did not absolutely, positively have
to be there tomorrow, you could get some good deals.

But the days when students could hang out at the airport and fly
standby for dirt cheap prices are long gone. The term "standby" is
still used, but doesn't mean what it used to:

   1. Most airlines no longer sell standby tickets at a special
      fare, but will sell you a standby ticket at regular fare. In
      other words, if they have space on the flight, the gate agent or
      ticket agent will sell you a ticket for the flight on the spot.
      All this means is you can buy a full fare ticket on the spur of
      the moment.

   2. Nonrefundable, nonchangeable tickets can often be used for
      standby travel on the same or similar route (sometimes with a
      slight surcharge). This most often comes up if you missed your
      flight (e.g., you arrive at the gate after the plane has left,
      and catch another flight to the same destination later that day), 
      but you may be able to use unused flight coupons in 
      this manner. For example, if you bought a ticket but didn't go
      for some reason (a reason that didn't entitle you to a refund), the
      ticket may still be usable for standby travel on the same airline.
      (Your best bet with such a ticket, however, is to have your
      travel agent or airline credit it towards your next ticket.
      You'll probably have to pay a fee, but it's better than throwing
      away the ticket.) All this is often at the whim of the gate
      agent, and is based on the notion that a ticket is often
      designated as being good for travel on the airline for one year
      from the date of issue. Your luck will vary by airline and gate agent.

   3. If you're on a later flight but get to the airport early,
      check with the attendant at the gate. You may be able to get on the
      earlier flight is there's space available (but this may result in your
      getting no "snack" or losing your special meal). This works even
      for "non-changeable" tickets. 

Airlines stopped the practice of selling standby tickets for a variety
of reasons:

   1. Even if a seat isn't taken, putting somebody in the seat still
      costs the airline some money (e.g., fuel). 

   2. Standby travel played havoc with airline load management programs.
      Airlines were sometimes faced with having to unload fuel to
      change the weight distribution of the aircraft because of a
      sudden appearance of several standby passengers.

   3. Some people would "ensure" the availability of standby seats by
      making a large number of regular reservations, and then an hour
      before the flight release the block of seats, virtually ensuring
      that standby passengers will get aboard at cheap standby fares.
      This caused airlines to overbook flights, and eliminated their
      ability to schedule their flights accurately. Since the
      elimination of standby tickets, overbooking errors have dropped
      significantly. (Another result of this practice is the automatic
      cancellation of unpaid reservations after 24 hours.)

   4. Airlines felt that they were losing regular business to standby travel.

Flying standby, however, may be making a comeback. In 1993, some
airlines had special one-day fares for folks traveling on Christmas
and New Years. These holidays are the nadir of the travel season, so
by offering these fares, the airlines were able to increase their
business on flights they had to fly anyway. (Yes, airlines still have
to fly the routes, even with empty planes, because scheduling is so
complex. For example, the plane in Atlanta might need to fly to Boston
so that it can be used on the Boston-Washington route later in the
next day.)  Now these holiday fares weren't true standby fares, but
the concept was similar. Since most flights these days fly at 85-95%
of capacity, one of these days an airline is going to get the bright
idea of reinstating standby fares in one variation or another. For
example, some airlines now sell discount flight coupon booklets to
senior citizens; the coupons may be used for standby travel. And of
course, US Air Force personnel can fly standby on some US Air Force
planes on a space available basis for dirt cheap prices, but with no
amenities (and maybe no real seat either, but strapped to webbing on
the wall).

Some foreign airlines still provide standby fares, sometimes limited
to youth.

If you are flying standby one way or another, here's some advice:

   -  Make sure you get to the gate EARLY. If several people are
      flying standby, you want to make sure that your name is first on
      the list.  Note that connecting passengers, bumped passengers,
      etc., get priority over local boarding standbys. 

   -  Days which are bad for getting bumped are usually good days for 
      standby seats. Don't fly standby on the day before Thanksgiving
      or the Sunday after, you won't get a seat. On Thanksgiving day
      itself, you're likely to find a seat.

   -  On really busy days it might pay to show up early for the
      *first* flight of the day, since standbys who don't make it can
      "roll over" to the next flight. 

Some airlines will only allow "same day" standby travel (i.e., you
overslept and missed your flight, and will take a flight on the same
route later that day). Others will not allow you to fly standby on a
flight if all the seats in your fare class have sold out, even if
there are empty seats available. If the agent at the check-in counter
gives you trouble, go directly to the gate and see what happens. 
[Agents at the check-in counter are under more pressure to enforce the
rules than gate agents, who have some discretionary power. If you can
avoid dealing with the agents at the check-in counter, (e.g., no
checked baggage), so much the better.] If you want to try flying
standby on one airline using another airline's ticket, bypass the
check-in agent and go directly to the gate.

Most airline employees (and their spouses and dependents) can fly
standby on that airline for cheap or free (as "representatives" of the
airline). Crew who must reach their destination for work purposes have
priority over all non-revenue passengers. Active employees have
priority over retired employees and dependents of an employee. There
is a pretty strict dress code -- suit and ties for men, and similar
requirements for women -- so you can't fly wearing a jogging suit.