Yeah, I've been sick in a plane many times, went full-out only once. Basically as mentioned above, good ventilation, and higher alt's help. Just remember, once you do start feeling sick, you can't just "get better." It's easiest to just go ahead and discontinue the flight and take a break. As a CFI I had a few students who thought they could do it........wrong! It's nothing to be embarrased about either, it's something that happens. After awhile you'll get used to the sensations and it starts feeling natural!