There are several medications that are excellent for motion sickness. Depending on severity the OTC calming agents mentioned and dramamine can work for many. Some of the more severe cases may need a stronger prescription medication, Cerenia, Zofran or metoclopramide.
If anxiety is the major issue, there are also prescription meds specifically for that, benzodiazepines (valium, ativan, alprazolam), clomipramine, prozac. As a last resort acepromazine (I've found most meds work better with less sedation).
The training or desensitization mentioned will help also.
Doug, DVM