I just usually rinse the straps and padding in warm water, while squashing the padding to work water in/out, then putting the helmet where it will dry quickly (usually above the water heater) so bacteria isn't encouraged to grow. Same goes for my gloves.

But if you really want it sanitized, you can do the above in a bucket with a mild bleach solution, say 1 oz. of bleach to 1 gal. water, and afterward rinse it really well.