In the past, I have been told that I interview well. While I am a pretty soft-spoken and shy person, for some reason I do really well in interviews.. Here are just a few tidbits that seem to work well for me.
1. Dress the part. I know you've probably heard this before, but attire is extremely important. Look professional. Even if everyone else in the office is wearing jeans, wear a suit, slacks, a skirt, a blazer; anything that would be considered "professional attire". Fix your hair, break out the heels, do your nails, wear makeup... Also feel free to add your own uniqueness to the ensemble - throw in a pop of color, wear an interesting nail color (just not too loud for the interview)...
2. Have a firm handshake (especially if you're interviewing with a man). I am a pretty small girl. People expect me to have a wimpy handshake, but I have made it a point to have a firm handshake. A strong handshake is important because it's one of the first impressions you make. I love taking people (mostly men) by surprise with a firm handshake. It definitely says that there's more to me than meets the eye.
3. MAKE AND MAINTAIN EYE CONTACT. I cannot tell you how important eye contact is. People are not going to want to hire you if you can't look them in the eye. And it's important to maintain eye contact throughout the interview. You appear sincere and truthful.
4. Enunciate. This one is especially important for those of us in the South. People down here mumble and have very strong accents. I make it a point to speak slowly, clearly, and enunciate my words (that way you don't have to repeat yourself).
5. Have several copies of your resume / letter of recommendation. It's important to have several copies; especially if you're unexpectedly interviewed by more than one person. And, this way, they don't have to make copies. (Also use the good paper)
6. Smile.. And relax. You want to look like you belong in that office. Even if you're nervous, try not to let it show. Take a deep breath, and relax. Smile, laugh at their corny jokes, do whatever it takes to show that they don't make you nervous (even if they really do).
I know it's not a lot, but these things have worked for me, and I hope they can be helpful to someone else!