I went through ten interviews this fall for co-op jobs at different companies so I've pretty much gone through what you're going through right now.
Tips:
1. Calm down.
2. BE YOURSELF! Don't try to be someone you aren't.
3. Know what the job/position entails before going to the interview. This is good information to have in case they ask you why you want the job/position.
3. Jot a few points down on a piece of paper that you'd like to bring up during the interview - questions, general information, ANYTHING - just make sure you don't bring in the sheet with you; try to memorize it.
4. You're always going to end up shaking someone's hand in the interview. If you're like me and have sweaty hands when you're nervous, make sure you DRY THEM before you shake the person's hand; no one likes sweaty hands. Wipe 'em on your pants or put some tissue paper in your pocket and wipe them on that before you shake.
5. Don't make stuff up - no one likes a liar!
6. If they ask you a question you really don't know the right answer for, ask them if you could answer it at the end. If, on the other hand, you have a general idea of the answer they're looking for, try and answer the question to the best of your knowledge. This will show them your thought process and your ability to analyze different problems.
Good luck!