Laugh at least once during the interview - at their joke or make your own: it shows that you are emotionally stable, and people with a sense of humour are much more fun to work with.
I know it sounds strange, but it works.

Also? If you don't have any interview clothes, go and buy some post-haste. You can afford it, especially if you hit a place like Ross. They sell stuff like that for cheap.

GOOD LUCK! You can do it!