Dish washing detergent - something harsh like dawn should do the trick. Just make sure you put on 1 or 2 GOOD coats of wax on immediately after you do this, as you don't want your car susceptible to environmental factors - air pollution, UV rays, etc. I would also suggest buying a clay bar and using it on the paint while the wax is stripped off to remove all the shit you don't want on your car, like tar or metallic specks.