Warning! Do not believe the rumor that you can put your helmet in the dishwasher!
From the Troxel helmet website:
"The heat and forces inside a dishwasher are too intense. While your helmet may look brand new when it comes out of the dishwasher, it may no longer protect your head in the event of a fall. Even the heat inside the trunk of your car is dangerous for the integrity of your helmet.
Trunk temperatures can exceed 160 degrees and that damages the helmet’s materials. The best way to preserve the helmet’s quality is to keep it out of the sun when not in use and store the helmet in a tote or carrying bag that allows drying while being stored."
Recommended cleaning tips from Troxel:
"Many ASTM-SEI helmets, especially those designed for schooling rides, have removable liners. Wash and soak the liner in mild, soapy water, adding 4-6 drops of bleach and then rinse it in a dilute alcohol and water solution. Then allow the liner to air dry completely. The drying is important and warns that it’ll only make matters worse if you put a not-quite-dry liner back into your helmet.
Methods for cleaning the outside of safety helmets depend on the exterior materials. Plastic schooling helmets can be wiped down with a soft towel and some soapy water. Allow it then to air dry. Then you can restore luster and shine with an application of a light wax protector like Pledge. Micro fiber helmets, such as the newer Euro-styled helmets, can actually withstand the same mild soap and water treatment. Make sure you also rinse them well with clean water to remove all soapy residue. Again, air-dry the helmet in the shade."