If your helmet has a permanent liner, clean it using a no-rinse shampoo.
Here's a helpful hint from Heloise’s illegitimate brother on how to really clean hair oil from your permanent helmet liner.
I have an inexpensive Fulmer three-quarter helmet I like a lot for vintage bike chivarees. The esthetics are just right and it's DOT-approved, but the liner doesn't remove for washing. It got a little greasy inside after awhile. I don't have time for do-rags or skull caps to keep the thing clean. When I sling a leg over, I want to go - but what to do about the natural hair oil that collects in my brain-bucket's fabric? My Fulmer's innards were starting to resemble Belstaff jacket fabric.
The other evening a solution dawned upon me, and of course I owe it all to Mom. In her last years on this coil, she "washed" her snow white hair in bed with something called "NoRinse" shampoo and it worked well. Her hair became clean and fluffy. I asked my wife if we still had a bottle of that stuff around. We did. I tried it on my Fulmer's innards. The directions say "Ready to use. Just apply, lather and towel dry. Leaves hair fresh, clean and odor-free."
That's what I did, and by-gum, it did - the liner became clean and oil-free! It took only a few minutes. "Used by NASA," it says on the bottle, and maybe it was. It comes from an outfit called CleanLife Products of Springfield, Ohio, but I imagine it's in drugstores, too. Now when I put my bucket on over clean hair, it's still clean when I take the helmet off! My freshly-washed hair was still clean after a day riding from Port Townsend, Wash., to the Backfire Moto and back in Seattle last week wearing that helmet. – Brooks Townes