Sometimes the easiest way to contribute to scientific advancement is not to be the scientist, but to be the lab rat. Research projects across the country and around the world need subjects for their test and control groups. The two means they have for getting subjects into their tests is by recruiting and by accepting volunteers.
Recruiting happens when the researchers target people with specific conditions, usually by working through a network of doctors who treat whatever condition they are trying to treat. The doctors recruit the patients and, if the patients consent, the doctors sign them up for the trials and are usually available to administer the treatments and provide follow-up care as part of the study.
Volunteers, on the other hand, take a proactive step to contact the research center or some agent in order to volunteer for the trial. I use the term "volunteer" loosely here, because in many of these studies, the volunteers are compensated (in cash, free medical care during the study, or both).
There are several ways to find out about opportunities for you to participate in a research study. The first (and best) is to ask your doctor. My doctor works at a clinic where they do clinical research, and she takes part in a couple of studies, one of which she considered signing me up for (until she discovered that the samples of a cholesterol medication she was giving me lowered my cholesterol by a massive amount in just two months).
You can also volunteer by checking with a local testing center, such as those run by Covance, or by searching for available studies at ClinicalTrials.gov, a site run by the National Institutes of Health. If you're looking for studies outside the U.S., Thomson Centerwatch maintains a list of actively recruiting clinical trials around the world.
Even if you volunteer directly, though, you should still check with your doctor before volunteering for any study.