The main thing for students to realize is that this is their chance to let the university folks get to know them personally -- how they are different from other students; how they stand out; why the university would benefit from their being a student there. One of the biggest mistakes students make is to rehash what's already on their applications and in their test scores and transcripts.
Here are some excellent webpages:
... especially, from Why Interview downward.
... especially, from Preparation downward.