What works for me (with my Arais) (and this is not original, I found it on the net and I wish I could remember the writer to credit him/her):
<<Holding your helmet with the open side down, wet the interior with a gentle spray until all interior surfaces and pads are thoroughly wet. Don't submerge helmet in water. Using a damp CLEAN cloth, apply a solution of mild soap or Woolite to the interior surfaces. (If your helmet really smells, try a capful of PineSol added to the soapy solution. If you get PineSol on the helmet exterior, rinse it immediately; it can cloud the finish). Rinse with a clean spray of water, again with the helmet opening down.>>
<<To dry, take a clean dry towel and blot as much water from the interior and pads as possible. Use Q-tips to clean / dry air vents. Place on dry towel (or dish drying rack), and direct air from a fan into the interior. Every once in a while shake helmet to dislodge excess water. Continue fan drying until all pads and surfaces are thoroughly dry (usually takes 6-12 hours). Now you can smell the car fumes, hen houses, and sewage treatment plants you've been missing.>>
<<We don't recommend the dishwasher technique. We do recommend that you start cleaning your helmet BEFORE it starts to smell. Helmet can be cleaned frequently if you're the sensitive type; or if you really liked to hang around the locker room in Jr. High, not so often. Other tips: Wax exterior to protect color / graphics. Once a year coat all plastic / rubber gaskets with silicon to prevent drying. Dry visor gaskets can cause a lot of wind noise because they don't seal well.>>
Hope this helps.