I personally use a solution of 70% eco-friendly dish soap and 30% purified water to clean my outsoles. First, wet with a spray bottle, scrub with a brush, and then scrub soap with a dish soap dispenser brush until clean. Re-spray to remove dirt and grime, then towel or blow-dry (on cold). Always brush with the lines of your shoes' outsole. I also keep a small set of tools that is actually an iPhone tool kit to get any hard-to-reach areas.