I'm having problems with air sickness--what can I do?
First of all, don't panic. Air sickness among student pilots is common, especially when doing the upper air exercises like spins and spirals. There are no known case at Langley Flying School where a student had to withdraw from flying because of air sickness. With experience, we become familiar with the sensations of unusual aircraft attitudes such as spins and spirals, and our bodies become familiar with those sensations. Everyone has a limit as to how many spins or spiral manoeuvres they can do before they feel nauseous, especially if you haven't done them for some time. The important think to do if you feel sick during training is to communicate early with your Flight Instructor—this is important because, if you keep doing aggressive aircraft manoeuvres after you start to feel nauseous, it will only get worse. Also, stay on the control as long as possible. Chewing gum sometimes helps, as does opening the cockpit vent window. Flight Instructors always carry sick sacs in their flight suits, so have no worries about embarrassment. Overall, be patient as you will "grow" through the phase, and talk about the subject with your Flight Instructor.
Further Reference: Wikipedia