my working preventions are:
fresh air, unless it is positively pissing down, keep the windows open, regular freezing blasts of air, make sure they are no too warmly swaddled...
keep them singing, talking and looking ahead( I get dd to check for deer with death wish)
do NOT let them snooze/get drowsy - this is an indicator for my two being on the verge of puking( I usually tickle my children if they show signs of sleeping, unless I know they are really tired, but then we just get prepared to stop for vomit when they wake up)
I give dd and ds ginger snaps/ very plain biscuits and water to drink (if necessary)
on long journeys, regular stops, often found dd was always sick after the first 45 mins of driving
these all work for me, my dd seems to be growing out of the car sick stage but I am still very careful, ds isn't as bad for it but he will puke sometimes...
we had a recent car trip of 1,500 miles (it wasn't a holiday!) and obv spent a lot of time in the car and neither were sick