One method is to mix equal parts white vinegar and water in a spray bottle, then sprinkle the solution onto the spots and let it sit for a few minutes before wiping it away with a clean microfiber cloth.  Another method is to make a paste out of baking soda and water, apply it to the spots, and scrub with a soft-bristled brush before rinsing with clean water.  For really tough spots, you can also try using a small amount of rubbing alcohol on a microfiber cloth. Whichever method you choose, be sure to work in small sections and avoid letting the solution dry on the surface of your car.