Sit straight, make eye contact, and avoid crossing your arms or legs. Listen carefully to what the interviewer says. People love to hire individuals excited about their company. Don't make the interviewer guess if you really do want the job. Don't be afraid to brag a little about your experience and talents, and how they will benefit the company.
Most people get nervous during a job interview.
Practice with a friend, or interview often enough to defuse the fear. No matter what happens, don't overreact to obstacles such as salary, location, or other issues. It is always possible to solve these things later if you leave the issue open. Most people are turned off by those who focus too much on money. Convince the company you can do the job and that you want the job before discussing money and benefits.