I have used Woolite like sakurama said. I mixed some Woolite in a bowl of water and saturated the inside of the helmet with a washcloth and scrubed it really good. You can dunk the whole thing in water since the hard foam is closed cell and won't hold the water in. I used a kitchen sink sprayer and worked the water in spraying upward into the helmet. Be prepared to get shot in the face a couple times. I set the helmet near a gas fireplace for a day or two to air dry.
You can also buy helmet cleaning products like this
http://www.proclean1000.com/helmet.htm , but I think it is used the same way and may leave a scent behind. The Woolite cleans really well and leaves the helmet smelling like new.