Don't drink any "tasty beverages" or eat fried food, etc - that's probably on the handout your OSO gave you, though.
It is a half-day ordeal - usually takes no more than 5 - 7 hours. You will have about 7 little vials of blood taken, a dental check and x-rays, a chest x-ray, vision tests, body measurements (for the cockpit), vital signs checked - pulse, blood pressure, etc, a hearing test, and you will be hooked up to an EKG (or whatever that machine is that produces paper with the squiggly lines and requires those patches to be placed all over your torso). When it is all said and done you will have an interview with a flight physician. He will go over your past medical history, etc. If you had any broken bones or surgery, it would be nice, although not required, to have that documentation with you. The best part is when he tells you about testicular cancer and checks for hernias...
Get lots of rest the night before. Don't plan on going back to work or anything - because your eyes will still be messed up from the drops they put in.
I think the Air Force is the only service that checks for prostate cancer - so you don't have to worry about some poor guy's fingers up your backside...