10. Washing in the sunlight on a hot day
There are several problems with this, but they all start with the fact that the sun is going to heat your car's surfaces and make the water evaporate that much faster. The water-and-soap combo is a lubricant of sorts that helps the mitt glide across the paint without scratching it, but also, as the water dries, the minerals within it wind up etching onto the paint. You know how hard it is to get rid of water spots in your shower? It's infinitely more annoying having those on your car, and you can't scrub to remove them.
What to do instead: Wash your car either in the morning or evening when the sun isn't at its peak, or buy a canopy if you don't mind strange looks from neighbors. In a pinch, you can have a helper constantly run water over the car -- it's not exactly an environmentally friendly solution, but it will keep the paint cool and prevent spots.
11. Using a regular towel
Normal cotton towels are neither very absorbent nor terribly soft. What's worse: they're moderately abrasive, so if you're not drying your car with all the caution of a neurosurgeon, you're inviting trouble.
What to do instead: Use a good-quality microfiber towel; because of the way it's constructed, it will hold more water before reaching its saturation point. If you get a good one (read: not bargain basement at the local big-box store) it won't scratch your paint, either.
12. And washing it in the normal laundry
Most laundry detergents have fragrances and softeners in them. You might think nothing of that, but once you dry your car with a treated towel, you'll notice a streak that reflects like a rainbow, thanks to the chemicals.
What to do instead: Wash your car towels separately, using fragrance-free detergent. If you managed to drop your towel on the ground at some point, go for an extra rinse.