Having been and interviewer recently...
Make sure you:

Know about the company and what they do, but do not come on too strong, a bit like with the opposite sex, it is quite off putting to have someone telling you what your company does and going on and on about it. Make sure they know that you know what they do, and that is enough - look at mentions of them from other places, rather than just their own website.

Ask lots of questions, but make sure they are interesting questions, maybe leading on from something they have said, rather than random questions, unrelated to previous comments.

Do not speak too loud or too fast. When you are nervous, you tend to do both, so however fast you think you are speaking, slow down.

Take up the offer of water, having a sip can be used as a stalling tactic so you can compose your answer in your head, as well as moisten your mouth, so you don't get toungue tied.

I hope these help, and that they aren't too late! Good luck!
(The Kwyjibo A big, dumb, balding North American ape., Tue 24 Nov 2009, 10:24, Reply)