left in first gear, on the side stand and if you can, front wheel up against the bulk head. Take a towel to put over your seat so the rope or the ratchett strap which should go over the seat to hold the bike in place. In the UK, cross channel ferrys also have wheel chocks which are useful as well. Never had the bike move.
Mac has given some good advice straight from the horses mouth as they say. Also this link might be useful with a bit more info about the trip itself. Specially the cool weather.