Here's another one I couldn't do without since I've used it. If you ride in cooler weather or want a quieter helmet this is the ticket. I bought the "cold weather" version and it really does the trick. It has a zipper vent in front too so you can really use it year round if you want and so far it has fit on every helmet I've tried.
http://www.mawonline.com/newsite/noj-helmet_warmers.htm#QUIET RIDER COLD WEATHER
http://www.mawonline.com/newsite/noj-helmet_warmers.htm#QUIET RIDER COLD WEATHER