I keep a couple of empty ice cream or washing capsule tubs in my car, you know, the plastic tubs with lids on. If my young 'un is going to be sick at least he can aim it into the tub. You then put the lid on and put it in a bin next time you stop either at home or en route. Make sure you get the next one ready after though! The smell of sick in a car on a long journey - yuck, yuck, yuck! Hopefully this little tip will help you avoid it.
Keep some Dettol or other strong smelling disinfectant in the boot too, along with some wipes, to mop up any spills and to mask the smell.
You could also take a couple of sick bags off a plane, next time you're on one, and keep them in your bag or car ready!!
Before you get in the car wipe down the seats with a cleanser that smells like lavender, its soothing to kids. Keep lots of zip lock gallon bags for older kids and for younger ones cut the top off of old gallon water jugs. Make sure you have pedialite or gatoraid on hand so the kids don't dehydrate and don't drive right after meals.