Get a helmet with removable liner. I personally don't care for the added layer of something like a skull cap or silk, or whatever they call it.
I sweat like a pig anyway and summers here in north Texas are both hot and humid. My old RF800 and RF1000 each smelled bad. After 2 1/2 years the RF800 was so rank I couldn't stand to put it on my head, so I bought the RF1000. After a year and one summer it, too, smelled awful, but I had the good fortune of dropping it and busting the rear vent system which forced a replacement.
So, in April of 2006 I bought a Shoei X-11 which has a removable liner. I wash the liner a few times each summer. I use my normal shampoo over warm water at the kitchen sink, wash the liner and cheek pads with my hands and watch the nasty brown water go down the sink. Then I rinse and wash with shampoo again until I see clear water go down the sink. Finally, I rinse thoroughly and then leave them out on a towel to dry. After three full summers and ~30,000 miles the liner and cheek pads are in good condition, and I can detect no odor at all.
One thought about price. While the X-11 is significantly more expensive than the RF1000, cleaning the liners will add life to the helmet because the liner won't deteriorate as fast. That, and it doesn't stink. Given that I'll actually wear it for 5+ years it'll come out cheaper than buying a pair of RF1000 helmets over the same time. In the future I'll only buy helmets with removable liners.
Just how old is your helmet that now that you've lost your hair it is loose?
Had the same thought. If you bought that helmet with a full head of hair, and you've used it up to baldness, I'd think that it'd be a real
old helmet by now. Unless, of course, there are other health issues leading to the hair loss.