For guys in the RAF who suffer from severe airsickness, they attend a "desensitising course" which involves going down the the Centre of Av Med for the RAF and doing theory and practical excercises to combat airsickness, starting benignly and building up gradually. This can lead to several hours in the back of a Hawk at Boscombe, where they take you to within an inch of your life and let you relax to build up your tolerance. Not pleasant at all, but generally very successful.
Airsickness can be reduced or even erradicated with increased exposure as has been mentioned. It appears that it can be more of a psychological battle than a physiological one at times, and if you think you are going to be sick then you probably will. Best thing when you get that horrible feeling is to relax, get your head out of the cockpit and focus a long way off and try not to think about it!!! Obviously there are those cases where this wont work but you have to be positive about it. It is always worse when you are not in control as well, which is kind of obvious.
I think there are some natural remedies for motion sickness, but these must be approved by a doctor before flying on them as they may have nasty side effects!