Congrats!!! I go Monday for an Oral Board interview myself. The only thing I keep telling myself is to just relax and be myself. If they don't like me enough, then it's just not meant to be.
In my limited experience, being myself has always worked best. You know why you want the job, so just tell them from your heart. I have friends who always try to have prepared statements, speeches, and responses memorized, and that just seems fake to me. Can you imagine what it sounds like to the people who do all the hiring for a living? I think it's okay to have a guideline to work with, but no more.
I remember being asked by the Psychologist before I got hired as a dispatcher, he asked me what I had to offer the department that makes me a stand out candidate. The first thing that came out of my mouth was a John Buccigross quote: "I'm clutch, and in life, clutch is everything."
It was hard not to laugh and I have no idea why I said it... but it worked. lol 
Don't be afraid to show your personality!
Good luck!!!
"You're abrasive, an idiot, a liar, a liberal and fool. I'd call that fairly accurate. Wait a minute. I think I could have just called you a Liberal and that would have included all the others ..." ro56 @ realpolice, 2006.
RIP Sergeant Reeves and Officer Gilner