Hi A.-
I too get carsick and it started when I was in elementary school. I ALWAYS got sick on the way to church. My mom realized that if I ate something really sugary for breakfast I got sick every time (Froot Loops- those things were TERRIBLE coming back up). Bland toast, etc. were the best things to eat if we were going to be traveling right away.
I also would suggest moving the carseat to the middle if possible. Looking out the side window makes things WORSE, because the movement of objects outside the window is disorienting. Objects in the distance don't "move" as quickly, so it's much better to look out front. Also, if I have an AC vent blowing DIRECTLY on my face, I don't get the sick feeling as much. It has been better the older I get, but turning around as a passenger to hand something to one of the kids is torture, as is looking at a map or digging in my purse.
Also, I would think that having a buckle/straps that are too tight across the tummy would make me (if I were a kid) more likely to get sick. I generally err on the side of caution, but if you've got a kid who is miserable, my instinct is to make the kid as comfortable as possible while still being safe.
As an adult, the Sea Bands have helped me considerably. I know pregnant women who have worn them 24-7 for months to help with morning sickness who swear by them. It's funny, but being on a boat doesn't bother me as much (probably because of the wind off the water). Being on an airplane doesn't bother me until they do the big turns after takeoff and for landing.
Sorry this is long.....
Good luck and keep a bucket and paper towels at hand. Also a bottle of water to swish and spit to get the bad taste out and acid off the teeth....
J.