Short Hair: Have a haircut a good week before the session. You want to look crisp, but not newly shaved!
Longer Hair: If clean cut isn't your look, then a little prep and styling so you know how it should look on the day is great.
Clean Shave: As well as the usual shave (use a new razor to avoid nicks) just make sure the difficult areas, like just under the jaw bone are well done too.
Stubble: If you are going for the stubble look, then don't shave on the day - we need more rather than less for it show up in photos.
Beard: Make sure it has a good trim - it is often worth getting a barber to do this for you. Shave any strays at the edges of the beard.
Good Sleep: The night before you want a good rest, this helps the face hugely, so no heavy drinking the night before!
Zits: If you wake up with zits, then just leave them alone. I retouch everything anyway and it is easier to deal with a small zit then the disaster zone of picked zit!
Smile: Men have great troubles getting a natural smile - women find it really easy. Just think of something that genuinely makes you smile. If the smile happens in the mind then it happens on the face.
Clothes: You want to choose 2 or 3 outfits for the shoot. Get them ready in advance and make sure they are all wrinkle free. Try and avoid logo's or text - that can be really distracting in photos.
Shoes and Socks: Remember to choose shoes and socks that go with your outfits.
Props: If you are bringing any props, instrument, sports gear, hobby stuff, get that organized in advance too.