I have always had motion sickness issues. I can't go on any of the "spinny" rides at the amusement park. On my first soaring hang gliding flight (first flight over 15 minutes) I started to feel really bad and eventually vomited. I eventually gave up hang gliding partly due to the rotten feeling I would get on any non-trivial flight.
Nausea can easily be made worse by too much head movement or looking in the wrong place during a turn. You help delay the onset or severity of nausea by using minimal or slow head movements, looking to the inside of a turn, and never looking down during a turn.
Several people have had success eating various forms of ginger such as ginger root pills. Even ginger snaps (commercial cookies or biscuits) have been helpful, to some.
There are all kinds of ideas such as ginger that people recommend to help with motion sickness but none worked for me. As a private pilot I came across an article about a device that you wear on your wrist like a watch but just the other side. It has a battery in it and gives you a mild electric "shock" to the nerves in the wrist. It REALLY works, at least it does for me. Here's an article published in a respected aviation news site:
They are not cheap - $129 as of this writing (May 2008) but to me it's worth every penny. I can now fly for hours not having to worry about keeping my head still and only looking certain places just to delay the inevitable. With the ReliefBand® I've been able to just fly - and enjoy it. I know sounds like a commercial but it's not. I've gotten nauseous on many flights and vomited on three - all when I wasn't wearing the ReliefBand®. So far I have had no problem at all (yeah) on flights that I was wearing the ReliefBand® (with working batteries).
The only downside to the ReliefBand® is the need for batteries. If the batteries go dead before or during a flight you are stuck.
Update: ReliefBand has been bought out and only a similar product is available. They are now much more expensive and you can't replace the battery. Once the battery dies you need to buy another one. What a shame.
As of the 2011 flying season I have switched to using Sea Bands. So far so good. Others have reported no success. But they are very inexpensive and do not require batteries so I strongly suggest trying them first. Especially now that the ReliefBand is so much more expensive.
At the end of the 2010 flying season I was flying in some pretty rough air for two hours. I noticed my ReliefBand didn't appear to be working later in the flight. But I wasn't feeling any nausea. So far at the start of the 2011 flying season I have not used my ReliefBand at all. But after a few flights I started wearing Sea Bands. I have had no trouble with motion sickness so far this year. I believe I am actually getting over my motion sickness issues - at least while flying hang gliders. I suspect my use of the ReliefBand over the last few years has sufficiently conditioned my brain to get over the problem. Though I need to experiment a bit by not using anything and see what happens.