Not sure if your comment is that you actually feel queasy, or if you are in fear of getting sick ... but here's my 2 cents regardless.
Everybody's different, but for me it was, initially, a combination of nerves and either wolfing down food right before lessons or not eating close enough to the lesson. I agree that a nice, nutritious meal that has "settled in" before you take off is the best thing. Too much coffee is not good, either, and you should be hydrated (just enough to not feel thirsty).
In the beginning, I was definitely far more likely to get a little "green around the gills" when the CFI was doing most of the flying... and if it happens to you, then you suddenly feel just fine after you get the controls, you will see how much of it is psychological. It's amazing, really. Your physical state (especially the digestive system) can make a difference, but it begins with a psychological state. There's a huge difference between anxiously waiting to execute some new maneuver and actually doing it. When you're doing it, you're too busy to get anxious (well, most people are, anyway). Or battling turbulence- hooboy, it really sucks to be a passenger in turbulence compared to having the controls yourself.
But the most common thing you will probably hear, and what you will probably discover yourself, is that in time it becomes less of a problem. So if you're merely concerned about it, try to put those worries behind you and just make sure you have your personal fuel and fluids in order before you fly. If you are already feeling sick sometimes despite being prepared and understanding the "psych factor", try to hang in here... you should overcome it in time.
Starting glider training, after over 200 hours in powered planes, re-introduced me to this whole nausea thing... first time I really dug in, trying to work thermals, I was doing most of the flying, but going around and around (and around!) in a pretty constant steep turn for almost an hour under a plastic bubble on a sunny day actually got me to the point where I said "this is not fun anymore, let's stop circling".
i was a little disappointed in myself, but I just got over it. Felt fine after that, and continued exposure to turns in the glider has me at the point now where I doubt I'll ever get queasy again. Until the next time I do aerobatics...