Like most have said, the majority of swirls come from improper washing & drying technique. Although it is next to impossible to prevent all swirl marks, there are steps to take to prevent as much as you can. I know of many people that use a leaf blower to dry the car after washing, The they go over it with a good quality microfiber towel & quick detailer. One thing that I always keep in mind, the less you rub on the paint surface without some type of lubrication (ie, soap or quick detailer) the less chance you have of creating swirls.