Before you clean your motorcycle pants, be sure to remove the protective pads that are inside. Get some Dawn Power Dissolver spray and spray it on your pants. Then let them soak in some water in the tub for 30 minutes. Use a bristle brush and scrub the pants, and then rinse them off completely. Next you should use a leather cleaner, and let the pants dry out afterwards. When they are dry, apply a waterproofing spray and put them in the dryer for a few minutes to let it work into the pants. When you are all finished, put the protective pads back in. The Motorcycle Journeys website has the full step-by-step instructions for this method.