If you're particularly nervous about your upcoming interview, prepare for the whole day.

Eat a good breakfast, your favorite if you have one. Lay out the clothes that you're going to wear and think twice, or maybe even thrice, if it's a good way to present yourself to the company. Print out your resume and other relevant documents. Know how you're going to get there and know how traffic it could be. Be there early.
Knowing the little details of that day could help you look at it as a normal day to get through and less like a serious ordeal.