Once a week, toss the bathroom rug in the washer. Wash it on the gentle cycle in warm water and let it air dry (especially if it has a no-slip grip on the back) before putting it back. Always follow the washing instructions on the tag if you're unsure.
You can also spritz them with white vinegar weekly to kill bacteria without damaging the mat. But, like your carpet, the detergent will help bring the bounce back to your mats.