Practice body language if it doesn't come naturally to you. For example, use gestures when you speak (shows enthusiasm and interest), don't cross your arms (makes you seem closed off and disengaged), don't touch your face or the back of your neck (this may make it seem like you're hiding something), make eye contact, but don't stare blankly (nodding and "uh-huh"-ing usually work fine - shows you're interested).
Interviewers, at some point, will typically ask if you have any questions, so have at least one or two prepared. If this is going to be a full-time job for you, questions like "What's a typical work day like?", "How will my performance be evaluated?", or "May I ask about your background of experience?" are fair. Even for part-time jobs, now would be a good time to ask about specific expectations and honest concerns.
Finally, never broach the topic of compensation or benefits until or unless the interviewer brings them up. If this is a set hourly wage job (and you already know the compensation), then you can disregard this. But if you will be paid "based on experience," expect a question like: "What do you expect in terms of salary?" Note that this is one of the toughest interview questions a person can be asked.
If this is your first job (ie, you essentially have no experience), try saying something along the lines of: "Money's a factor for me, but it's not my primary concern. I'm more interested in knowing what it will take for me to be successful at this company," or, "I feel that *you* have a more accurate sense of what someone with my qualifications is worth in terms of compensation. I trust that your offer will be fair." If the interviewer presses you on this, feel free to answer the question with a question ("How much does a person with my qualifications typically make?").
Also, if you haven't done so already, google "interview tips". There's a wealth of knowledge on the subject written by people who have boiled down the interview process to a science (or art, as the case may be).