Earlier posters did a good job of describing the route to the left (or right) seat in the big companies. For what it's worth, here's what I've found in the smaller North American outfits.
FTP school and military experience are a good way in. Employers will look for some sign of interest in, and experience with, little airplanes. Ex-army pilots and people who built a little airplane in their garage have done well. As Genghis said, we're in it for love, not money.
Most FTEs, and a few FTPs, work their way up(?) from the engineering department. Myself and several other FTEs started in ground test and/or performance calculations, earned a pilot's license at our own expense, and then wormed our way into the cockpit anyway we could. Be prepared to take the !!!!ty jobs and bad shifts along the way. Again, you need to show some extra interest and experience in aviation. Don't show up for a 9 to 5 office job in engineering and expect to be noticed. From university to the first occasional flight test ride took me 4 years. Full time employment in the test department took another 10 years.
As an extreme example, the experimental pilot at one light airplane manufacturer I worked for started as a laborer the layup department, with a diploma in a non-related field. He read everything he could about aeronautical engineering, got his license at his own expense, and instructed on ultralights on weekends. After several years, he was promoted to technician in ground test. From there he worked up to assembly line QA, still helping out in test on his own time. The next step was production test flying, and finally, after a few years, helping out in flight test (while still working QA, helping in test, and instructing on weekends). When the experimental pilot retired, he just slid right in to the empty seat. All in all, a remarkable story of patience and hard work.
[ 30 January 2002: Message edited by: Weight and Balance ]</p>