ProGold Bike Helmet Cleaner & Deodorizer
Just spray this aerosol foam all over the dirty parts of your helmet and let it sit for about one minute. Then, wipe clean with a rag.
You can also use this stuff to get rid of odors in your helmet. Same deal: just spray the foam inside your helmet, let it sit, and wipe clean. ProGold Helmet Cleaner and Deodorizer removes the root causes of foul odor without damaging helmet liner and padding.