After washing your car, it is time to clean the microfiber cloth. If you are just giving it a quick cleaning, you can hand wash it with any mild hand soap. Dish-washing liquid works well also. It is important to thoroughly rinse the microfiber cloth after washing it with soap. In order to get it completely clean, put it in the washing machine. It should be washed separately from other towels or clothes, in hot water. Hot water expands the microfiber, which allows trapped dirt to wash away. Do not use fabric softener or liquid bleach. Do not put it in the dryer, but rather air dry your microfiber cloth.
For more info, see How to Do a Professional Car Wash at Home