Job interview tips

UPDATE 2: Heard back. I was unsucessfull. I'm absolutely gutted.

UPDATE: Had the interview yesterday. I think it went well. I don't want to get my hopes up though. I should hear if I got the job today.

Hey guys. I normally don't like to talk about my personal life on Giant Bomb, but I want some advice and I thought I might as well give you guys a shot. So there is this girl I like... OK, that was a bad attempt at a joke about girl advice threads... Moving on...

As any of you following my twitter account will probably be aware of, I had a telephone interview for a job today. It's just for a call centre job (outbound sales), but I need a job. The phone interview was successful but only just. I thought I was confident, but as soon as that phone rang I turned into a nervous jittery idiot. At the start of the role play I totally messed up and stuttered and asked if I could start again. I think I managed to show some potential, which is why they've given me a proper interview tomorrow, but if I can't improve tomorrow I'm never going to get the job. And since the telephone interview went relatively badly, now I'm super nervous about tomorrow.

As I understand it, the interview tomorrow consists of some in depth questions followed by a role play in which I'm given some details about a product/service they sell and I have to act as if I'm the salesperson and sell this product to the interviewer. I think I can do it, but I'm worried about getting nervous and making some big mistakes and screwing up.

Now, I'm basically looking for any interview tips you guys have. Particularly about calming your nerves prior to an interview, but anything you think might come in vaguely useful would be greatly appreciated. Thanks guys.

EDIT: OK, one of the other problems I have at interviews is the part when they say "do you have any questions for us?". Apparently it's just as important to come up with some questions to ask them as it is to answer the questions in the interview properly. I always prepare a few questions prior to the interview, but they always answer them in their introduction talking about the company so by the time they ask me for questions, I have nothing to say and I'm forced to give a cop out answer of "No, you've explained everything really well". What are some good questions to ask them?