$$ Volunteer to be a Health Clinical Patient @ NIH Bethesda, MD

Visit the site and then call the number and you will immediately start getting contacted about being a healthy volunteer!
Don't worry - you get paid (a lot)!

Most studies will go like this:

1. Screening phone call
2. You go in and get tested / screened and paid for that day.
3. You come back and do studies across multiple days.
4. You receive a check and free food everyday you're there!

http://www.cc.nih.gov/participate/studies/healthy_vol_prg.shtml