I partially agree with the others about motion sickness mainly being a psychological thing, however as I have found out the hard way certain illnesses and ailments can have a huge impact upon your tolerance to air sickness. For example I was extremely rarely airsick until I had glandular fever in 2004, an after effect of which was that my acid tolderance was reduced to a much lower level than it should be. I found that when I started my PPL training I was feeling unwell on pretty much every flight as pretty much anything caused my stomach to flare up. I even had one experience when solo that very nearly developed into a dangerous situation as I was feeling too unwell to concentrate on landing the aircraft properly. It's only now after 80 hours flying that I find that my tolerance to airsickness is at an acceptable level. If you do start feeling unwell, something that I finds helps me (only when as a pax though) is to basically tell myself exactly what is going on, tell myself the motions that the aircraft is making and I find I can normally hold the sickness at bay. Give it a try next time you feel really bad!