Best way to assist in preventing it is to give the car a wax next time after you wash it. Once I have washed and rinsed my car I take the hose fitting off the end of the hose, turn the pressure down and run it lightly over the car. This should remove most of the water droplets from the car. Its kinda hard to explain. If there are a few left over I place a chamois over say the roof of the car and pull it slowly towards me. You can also invest in a good chamois. I have a kenco which you can buy for around 30 bucks