Umm, seriously take it in the shower with you. No kidding, I've been doing it for years. In Florida I used to shampoo my lids in the shower about every 2 weeks.
It's recommended by Arai and it works like a charm.
No mystery chemicals in your helmet, no expensive contraptions, just a good luke warm shower. That's about the only caveat, do not use REALLY hot water if you take REALLY HOT showers dial it back a bit.
Fill your helmet with water, drain ad your favorite shampoo, agitate gently, rinse well, allow to dry on a towel for a few hours or overnight depending on your environment. Your helmet will smell great, and last for years.