As one who has had to medically manage aircrew with airsickness
, as well as having had the problem myself at the start of basic RAF flying training, I came up with a few rules of thumb. The first is that you keep yourself as physically fit as possible and keep alcohol consumption and cigarette smoking to the minimum - both affect the balance organs in the middle ear. Eat sensibly - nothing too rich, and little and often rather than big meals infrequently. Keep caffeine-containing drinks such as coffee, tea and cool drinks to a minimum. Try and get into regular habits of sleep so that you wake up in the morning bright-eyed and bushy-tailed. The psychological approach is most important as it is a vicious circle. The more you dwell on your apprehensions, the more likely it is to happen. Try to cultivate a positive outlook. It might help to remember that your instructors have seen it all before, and might even have had the same problem themselves. When I had a recurrence of the problem for a few sorties during Operational Conversion to day fighter/ground attack where we seemed to spend more time upside down than the right way up, I found that it helped to say "Do you mind if I fly it for a bit?" That usually worked as the instructor would not appreciate the product of a technicolor yawn sprayed all over him. Also, when you go on your course try o ensure that you are in current flying status if possible. That's about it. Good luck!
