Whenever I notice my helmet getting a bit funky, I throw it in the dishwasher, either with the dishes if there's room (I live and eat alone, so in that scenario I don't consider it nasty, especially as I use the sanitize option) or by itself. I dry it in the sun, and it is squeaky clean until the next ride.
This procedure no doubt accelerates the wear-out of the foam pads that interface between your cranium and the helmet proper, but these are easily replaced. The rest of the helmet seems none the worse for it. Lately, I also remove the straps and wash them separately in the laundry.