I had the same issue with my older dogs (lived till they were 14), my new pups have a hard time as well. Since they usually travel in the back seat with leash belts, I got into the habit of removing the seat cushions. It lowers the height by 3-6" and they have an easier time jumping in and out.
_________________________
member of http://www.soxc.org
member of http://www.nexterra.org