I've had this four times now, certainly opening air vents, getting them looking off in the distance and keeping them talking all helps to stave off the barf, but sometimes they are just going to chuck up whatever you do.
One lad on a trial lesson, halfway in he brought out his own sick bag, and then announced that he always gets travel sick before he proceeded to fill the bag to the brim. Thanks for the warning
Sometimes there's nothing you can do to make the ride smoother, so it's more about dealing with the aftermath and getting on the ground within a reasonable time.