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.