This is the reason why my 2 1/2 year old DS is forward facing, while his twin sister, who is nearly exactly the same height and weight, is still rear-facing. I just couldn't keep cleaning up vomit every time we drove anywhere.
I myself am subject to severe motion sickness, so I know how it feels, and I couldn't put my DS through that anymore. I also couldn't keep cleaning vomit off the entire car AND my other two kids, and I couldn't deal with the stress of having to concentrate on driving while hearing poor DS be so miserable.
I also wasn't willing to regularly drug him, nor was just staying home an option for us.
So I turned him, and he still does get motion sickness occasionally if there's a lot of twisty turns or stop-and-go traffic, but it's so much better than it was.
What can help, if you are coping with motion sickness? Lots of fresh air-- open the windows. Encourage him to look out the window, not at some close-up activity like a toy. If the child is old enough, candied ginger or ginger ale can be a big help. Noticing the signs, and getting the child out of the car to sit or run and play on solid ground for awhile, helps a lot too. Light snacking is helpful, too. And sleeping in the car seems to help prevent motion sickness well, too, so doing your driving at a time when the child is sleepy is a big help. Those bands that you can wear don't seem to fit small children very well, so those haven't been helpful to us.
And just in case all else fails, you'll want to keep a cleanup kit in the trunk, with changes of clothes for the child, you, and any other children in the backseat with the child, towels, rags, bags, disinfectant or other cleaning product, something that can deoderize, etc.
me , he , my three , , and -- and the one we lost