I'm a little bit overweight and up here in the tropics I suffer from sweaty hair and wet helmet linings with the consequent smelly helmet syndrome.
Bought a Sharp Helmet Ion Generator, you just plug it in, put your helmet over the outlet, and leave (overnight). It supposedly blows a stream of ionised air through the helmet which kills the bacteria eating the sweat and leaving smelly residues.
Since using it, my helmet doesn't smell after getting wet from sweat, in fact the flow of "ionised" air through the helmet dries the lining as well. I don't know if it's the drying effect of having air flow through the helmet or if it is in fact the ions killing the bacteria and mould, but my helmet is much fresher now that I put it on this machine after a medium/long ride (will have to put it on the machine even after short rides soon, it's getting really hot and muggy up this way).
