There are the usual options of flying standby and off-season and such...
And, there is flying as an air-courier. Basically, you fly with only what you can carry on-board. Your "luggage" is checked in and picked up under your name (you may not even see your "luggage").
Never have done this myself--but it did seem like you could really save some big bucks if you are able to travel light and follow their schedules.
Normally, it is for one ticket only and if you have somebody else that you want to travel with--generally, they will have to fly a few days earlier (or later).
You have to be, at least, 18 (sometimes 21) to fly under these tickets.
-Bill