That's a big problem for me too. I've flubbed some pretty good opportunities because I got nervous.
A few things to consider:
1. Make sure you're prepared. Have a list of questions ready. Go through the job description, and think about what you're going to say if they ask about any of the points. Know which points are your selling points, and know how you can spin any weakness.
2. Have the right attitude. This might be an opportunity it might not. You're there to see if you're the right fit for the company, and the company is a right fit for you. Don't be desperate, you don't need the job (nothing terrible will happen if you don't get it), you want the job, working is fun and rewarding for you, convince yourself of that. Remember the employer needs someone to do something for them, and you're the helpful dude who gets that and wants to fulfill that need because it feels good to be helpful.
3. Make sure you're relaxed and prepared. Have your resume printed and preliminary research done as soon as you know you're going to be interviewing. Keep your Interview clothes ready at all time. If you had an interview, could you grab your clothes out of the closet and go right now? Make sure that answer is use. Do all this so that before the Interview you can be relaxed. Take part of the evening before the interview to go over your questions the company, and how you're going to answer the questions. Take the rest of the evening off to do something soothing and fun and get to bed early. Make sure you're well fed, well-hydrated, (and for me well caffeinated) the morning of. Don't rush, make sure you look good. I budget 30 extra minutes to get there because traffic and finding the building will eat 15.
Those will all help keep the nervousness at bay. But of course, you might still find yourself noticeably nervous. It's true for me too, that leads to the last.
4. Own it! You're excited for a new opportunity. It's like when a girl likes you. You get nervous. When I get nervous sometimes find myself starting to
clutter, I pause for a second, and say, "Sorry, I get little nervous when I'm excited". It's no big deal, you're still in control, you're just a little nervous. Most people get nervous before interviews.
BTW if you (or anyone else) want my opinion on something regarding interviews, feel free to PM me and I'll might over a few comments (I can't guarantee it though, I get pretty busy at times).