When you've finished waxing your car, don't throw the applicators out! These applicators are a high quality German foam and can be used many times.
Simply grab a small amount of Microfibre Wash (10mL or so should be plenty), and pour it straight into the applicator, then rub it in with your fingers under some warm water to loosen the wax from the foam. Rinse thoroughly until all the wax and Microfibre Wash is gone from the applicator, then rest them on a shelf somewhere, away from dust and dirt, on an old microfibre cloth to absorb any left over water in the foam. Lay the bigger applicator foam side down so that the cloth absorbs as much water from it as possible.
Next time you go to use the applicators, make sure they're dry, and give them a quick brush off so that there's no dirt or grit in them before using them.