I get asked this question a lot. The best advice I can give is to go get an Engineering degree (preferably either Aero or Mech). All test pilots I fly with are engineers and have come up to the Test Pilot ranks from one of two ways.
1) Get an engineering degree and join the military. Fly routine military stuff for a while (years) and then go to a Test Pilot School. From there do some military testing and then join the civilian ranks.
2) Get an engineering degree and join an aerospace company. Work on your PPL, IR, CPL, ATP with the profits from your highly paid engineering job (Hee, hee ..). Transfer into the flight test department as an FTE. Work with and impress the Test Pilots (will take several years). Get lucky and get selected to fill a vacant production test pilot position. Do that for several years. Impress the Engineering Test Pilots. Get lucky and get selected to fill a vacant engineering test pilot position.
I'm not saying these are the only ways to get a test pilot's job but in my company, most pilots got to their current positions by Method (1) or (2) and the break down is about 50/50 which I initially found very surprising. Either way it will take a long time - you need the experience as well as the skill.
[This message has been edited by sets (edited 31 October 1999).]