I don't get motion sickness, but I think I can offer some advice for those who do. First, always do these rides on a relatively empty stomach. Leave at least an hour or two, depending on how fast you digest, between your last meal and getting in the ride. I digest pretty slowly, so after a meal, I always steer my party to shows, sedate rides, or walk through attractions.
Better yet, get all the more intense rides out of the way before you have a meal. Then do the more sedate stuff afterwards.
Also, if you know you're on a ride that you find rough, hang on! Seriously. If you hang on tight to the handrails, it definitely smooths the ride for you. I do this on certain rough coasters. Of course, try not to hang on so tight that you end up hurting yourself.
Also, for most people, it will help to keep your head against the headrest, and look straight forward.
This is common sense advice, but if you keep them in mind and plan your day accordingly, they could help. And skipping a ride altogether is always an option. I always skip the Hulk coaster. Not because of my stomach, but the ride does give me a headache.
P.S.: Some people mentioned the upcoming Kong ride. I've heard that it will have at least one giant AA!