Q&A: rear-facing toddler carsickness

Jessica writes:

"Hello! Thank you very much for having such a helpful blog--I refer to it often, especially when we're suffering from sleep or teething troubles.  I have a question that I just can't find an answer to. My 16-month-old daughter is still rear-facing in the car (she's 18.5 pounds), and we're hoping to keep her that way as long as we can. However, she's been getting carsick whenever we travel for more than a half hour or so. Yesterday, I had a feeling she was about to "pop"--the car was warm and sunny, and she was crying a little bit--not mad, "I WANT OUT" crying, just "I don't like this" crying. So we opened her window a crack and tried to cool down the car with the A/C. It didn't work--without fail our car smelled like yogurt. Also, before we got in the car, we took off her coat and pulled her sun shades down to keep her cool(ish) and to keep the sun off of her face. Do you have any better ideas on how to keep her stomach contents down when traveling? (My husband and MIL think the only way to fix this is to turn her around--but I'd really like to avoid that.)  Thanks very much!"

I have no idea. Truly. I get carsick myself sometimes, and have no cure for that. And since I don't have a car, I really don't have any good advice for people who drive all the time. (I have lots of good tiips for taking kids on the subway, though.)

Readers? You must have some magic cure. Or is this some thing people just suffer through that I never knew about?