When I used to flw aerobatics people would always ask what they should eat before they go up. My answer, BANANAS. And it isn't because they help with any nausea or anything. It's because they taste exactly the same coming up as they did going down.

But seriously, don't not eat before you go flying because that will make you not feal well and make you perform poorly but dont eat a huge meal either for obvious reasons. A light meal or snack shortly before flying will help prevent motion sickness as well as you flying the airplane when you begin to feal ill. Ever notice how the driver in a car usually doesnt get car sick even when the person is prone to car sickness? It's because motion sickness in general is mostly psycological and if you are occupied doing something else, you aren't thinking about getting sick. So when the tummy gets twisy, take the controls, look out at the horizon, and fly the plane strait and level towards it.

Good luck.