Once you've been accepted, the first thing that you'll probably do is sign a consent form. This is a contract that explains the procedure and duration of the experiment, the possible dangers or side effects, and the compensation (i.e., how much you'll get paid).
Some of the simple experiments may not entail a full consent form (depending on what the questions are about), but always ask about it anyway. Once you sign, you acknowledge that the responsibility for any risks, and most of the liability, are put on your shoulders if something should happen. Read the consent form very carefully and ask any questions you may have.
After that's taken care of, just listen to the doctor(s) and do what they say. You're allowed to leave the experiment at any time, for any reason. And if at any time you feel sick or in pain, speak up immediately.
During interviews or questionnaires, your personal information is extremely confidential, and your answers will be coded (that is, "hidden") anyway.
After it's all over, if your compensation is a small amount, you'll probably get cash; if you're making more, a check will be mailed to you. Also, if you had to pay for transportation, gas or a babysitter, you might get reimbursed, so ask about it. If you took any drugs that might cause drowsiness, you may want to ask a friend to drive you to and from the exam (especially if it's an alcohol trial).
