Often you and the other interviewees do a lot of waiting around before you actually interview. One good way of relaxing is by just talking with other people. Getting to know a fellow applicant in this context will get you warmed up to talk about yourself with your interviewer and keep your nerves from mounting.
Consider making a checklist of stuff to leave home with. This is especially important if your interview requires staying overnight away from home. Everytime that I have an interview I forget one stupid thing (belt, deodorant, toothbrush). It's annoying and can potentially multiply your nervousness.