Also try using anti-seasickness bands - they operate on a nerve inside the wrist which actually does seem to make a difference for many people, so long as they're correctly positioned. (Acupressure/shiatsu uses the same point for morning sickness.)
I agree with MJ about having a reasonably full stomach, but I'd add stay as cool as you can comfortably be. Being too warm is never helpful where motion sickness is concerned.