I used to get very carsick as a child, and we went on a lot of family vacations. Things that helped were locating me next to a side window that was several inches open for fresh air plus running the A/C very cold, a hard candy like a Peppermint to suck on, and Ginger Ale or "flat" soda as mentioned. Also nibbling on plain old crackers (not junk food or sweets - just made it worse.) Focusing on a spot on the horizon sometimes helped, but it's hard to get a young child to understand what you are talking about.
Reading was a HUGE no-no (and I was a bookworm) as it would definitely push me over the edge toward "Linda Blair" style hurling everytime.
I personally couldn't handle the dramamine as it made me sick, but some others swear by it.
I'd also suggest traveling at night as much as possible. If I could get to sleep, I could usually do ok. If there's anything you can do to avoid stopping once your child is asleep, boy, would I try to find it.
Best of luck; it does take the fun out of road trips when your children are so uncomfortable. Hope it gets better -- thankfully I outgrew it (so sorry, Mike! I remember those days all too well.) Hope things get better for you.