Trust me, the more relaxed you are, the more confident the potential employer will see you as being. In most positions, this a huge plus. Getting relaxed is not always easy though.
Being a former IT slave from the dot-com (and succeeding dot-bomb) era, I had my share of interviews. After a while you develop a routine, and every interview becomes so much like the other, it's not even funny. What changed it for me, was viewing the process as Tim said above. Who knows, maybe you don't even really *want* to work for a particular company - and this is your opportunity to help relax. Approach the company with some skepticism. You know what YOU are capable of, but is this employer going to let you shine, or do they simply want another slave to add to their army of minions? Are they going to give you what YOU want?
At least this worked for me. Once I managed to get in the door to an interview, job offers would arrive within a week or two in about 95% of the cases. Now, getting to the interview was always a tough area for me. I could never really figure out the specific combination of key phrases and buzzwords that would pass the HR drone's filters.