Do you have one of those kids that tends to get carsick? If they do Laurel (a.k.a. Roadtrip Mom) has some helpful hints. She actually has a variety of ideas ranging from peppermints and ginger snaps to actually sticking your head out of the window. (I've done that when I was really drunk before. I swear it helps.) She also suggests keeping a tub of wipes and an extra set of clothes handy.
Laurel had a contest (which I missed when I was slacking last week) and her readers came up with some hilarious stories about their kids puking in the car. You should go check them out.
My kids don't get carsick that often, but since the boy has a tendency to barf a lot I try to keep extra clothes around anyway. This came in particularly handy when the twins came down with some sort of stomach flu the day we drove from Atlanta to Washington D.C. Ick. I wish I would have had the Fabreeze that day.