This will shock you! The static build up is in YOU not your car(s), and you discharge it against anything metal, like your car when you exit. Everybody can store static electricity to a certain degree and the severity depends on your serceptibility, the humidity of the day and (usually the main one) the clothes you are wearing. Nylon against wool is a good static charger (remember experiments at school with a lambs wool cloth and a beaker) and thats what is in most clothes and car trims are made of.
The best solution is, when you are alighting from your car hold your hand on the A pillar just before you put a foot on the ground. The rubber straps I believe are an old wives tale.