A friend of mine told me to do the following:

1. Get in the shower and get your hair lathered up with shampoo
2. Set the helmet on your head and smoosh it around so that you get shampoo all inside
3. Lather up the helmet straps
4. Rinse and let dry

I thought this sounded a little weird, but asked at my LBS. They asked the Giro rep who said "that's a great idea". Now all the guys at the shop do it. Apparently, the shampoo doesn't damage the foam and will go a long way towards getting rid of the salt and grime.

Cheers,

Susie