I have been instructing for less than a year now myself and despite having the odd queasy student, nobody has blown chunks on me yet.
With the ones that have come close (generally trial lessons) I tend to open the vents/apply cabin air. Try to get them to concentrate on something like flying the aircraft, or as a last resort get them to close their eyes while I return to the airfield by the shortest possible means.
As homeguard says, it's caused by the difference in sensations percieved by the eyes and semi-circular canals in the ears. If you get them to shut their eyes, the possibility of them being sick is reduced.
I have flown with a student who suffers quite badly with airsickness. He has one of the anti-sickness wristwatch style devices and it seems to work with him. When the time came to do steep turns, I thought my luck had ran out. However, he was absolutely fine.
