There's a magazine published by people who do these tests for a living. Some reportedly make as much as $50,000 a year, but they're driving all over the country to show up for different clinic's qualifying tests and they maintain as "normal" a condition as they can muster (or at least the clinician's perception of normal). This means you can't smoke, drink much, do any drugs anytime close to the tests, and you need ideal body weight and healthy readings and medical history on all the basics (though this varies by clinic and pay grade). The magazine's motto is "work is for suckers." They claim to have a lot more free time than most career people, but to keep it up their schedule is totally at the mercy of the clinic's. It's really easy to be disqualified for a high-paying trial, too. For example, if you even so much as have a couple drinks within three days of qualification tests your liver enzymes are likely to be elevated beyond their criteria. And yes, the people that make that kind of money say it requires them to lie constantly (never depressed, not even any mild allergies, etc.). Still, even if you're disqualified you get a bunch of medical tests/reports for free (liver enzymes, EKG, STD tests, white blood cell count, blood pressure). For most people it only makes sense if your temporary unemployment just happens to coincide exactly with a time the test and trial is conducted and you have nothing better to do (you're stuck at a clinic with no visitors for weeks on end for some). Payment is only in full if you complete the maximum number of days in a trial, and they can kick you out at any time. The check comes after follow up interviews or in segments as interviews are completed. Oh, and of course there's the whole thing about being a guinea pig to test the tolerance levels for a drug that may kill you or fuck you up in some other way if that's what the trial is for. If you ask the right questions you can get a good idea of the drug class, mechanism of action, and what other drugs it might be related to in order to better assess your risk, but they don't just give the info out without being pressed.
Yes, I attempted to do this once during a summer while finishing school when I was going to be moving far away in a couple months (at the protests of my girlfriend and family, heh). I made it through all the qualifying but then got rejected because my white blood cell count was elevated during the last test (never did get sick or anything, though - the tests were just oversensitive so I got fucked out of it).