For those susceptible to motion sickness it is advised not to eat anything heavy before the trip, nor to drink alcohol. Something like toast or crackers can be helpful.
When the signs of motion sickness start, like excessive salivation, cold sweat, headache, queasy stomach, and such, deep breathing and relaxation exercise might help. What I also used to do is leaned my head back, closed my eyes, and did not move if at all possible.
I have read that the most susceptible group are children over the age of two, while those over the age of fifty are least susceptible. Women tend to be more susceptible then men, and during pregnancy motion sickness might be even more pronounced.