Nerves during intereview

 
I would look into Toastmasters.  It is a great way to build your confidence.  Having to think on your feet and speak in front of others is a crucial skill and will help in an interview.  Of course you should be prepared by being able to speak to your accomplishments and understand how your can contribute.

I think there are two aspects to overcoming this, possibly three:

1. Preparation: if you are well prepared and know what you are going to say in advance, that's a big start
2. Practice: practice alone, with a video camera, and with friends to get used to saying your answers over and over and over. As SCM2423 mentioned, Toastmasters can help by giving you a forum to practice speaking in front of strangers
3. If 1 & 2 don't work, consider therapy. You may have deeper issues that a professional can help with.

Interviewing is one of the most critical skills out there if you want to move to another company or if you ever get laid off and need to find a new job. I think it's great that you are looking for ways to work on this!

TomW is correct to recommend Preparation and Practice.
The Interviewing Series offered by MT is an excellent resource for interview preparation. I have the interview series and recommend it to others. At $150 it is a bargain. Follow the counsel in the Interviewing Series and you will have a new confidence going into any interview. The structured answer format helps you beyond employment interviews by preparing you to answer questions quickly, succinctly, and in an organized fashion.
 

I'd agree with the advice above (except TomW's #3: you don't need therapy!)  
TLH is right about the interview series - follow the advice in those casts, and you will be ready.   
John Hack

I have consistently offered my friends and coworkers the opportunity to "mock interview" and I'll either do in-person or over-the-phone interview.  I'll ask all the normal, usual questions first and then wrap back around with suggestions, tips, and so forth afterward.  Folks have found that having the conversations with someone they know already and are comfortable with and getting that feedback that they'll never get from an actual interview (Don't stammer, use verbs more than adjectives, etc.) is invaluable.  They go into the live interview with a sense of "yeah, I just did this so I know what to expect."
Another tip is to ask yourself the questions (at home) and practice replying to yourself (and any other inanimate objects you'd like.)  Get in front of a mirror so you can practice making eye contact.  Practice moving your gaze from your mirrored self to the toaster back to your mirrored self and then over to the blender, etc. 
Prep GOOD!  I usually look at the job description or whatever information I have about the job and find the key areas they're looking for: driving change, identifying root problems, establishing/enhancing efficiencies, etc.  Then I'll come up with my SODAR: Situation, Opportunity, Decision, Action, Result.  I'll actually come up with a Word document that gives the five bullet points for each of the key areas.  Then I'll use those notes to practice my answers - in front of the mirror, in the shower, on the commute, etc.  I get to the point where I've got it nailed concisely but with detail, making sure to cover all five bullet points. 
The more practice you have, the easier it becomes to handle the curve balls that might come your way in the interview process. 
Also, BE YOURSELF!  It's not an interview.  It's a conversation about something of mutual interest to everyone involved.  Have the conversations as if you were at the dinner table with these folks.  (Maintain professionalism - don't pick your teeth with your knife - but relax as if it's a comfortable meal with friends who share a common interest.)
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that's why I said "maybe" and "If it's something that isn't cured by lots or practice and preparation".
I have actually met a couple people in my career who had severe anxiety issues that were corrected with professional help, which is why I mentioned it.