Honestly, 99% of it is mental. One time, it was a little bumpy during the flight, and combined with the awkward smell of the cabin heat, I got a little queasy. I didn't toss my cookies, but for the entire next day, I felt sick until I got into the cockpit again, got up, and did a flight without getting sick. After that, I felt good as new, and haven't gotten sick since (and have flown in some chop worse than that which initially induced the feeling). That flight convinced me that the majority of it all is your mindset. Chew some gum, eat your gingersnaps, do whatever helps, but when push comes to shove, if you think you're going to get sick and dwell on this thought, you probably will.

EDIT: Maybe this can help.
http://airlinepilotforums.com/showth...ht=airsickness