I've been wearing a cap under my helmet for years now. I started doing it more to keep the sweat from dripping in my eyes as opposed to saving my helmet. The added bonus too is that if any bugs fly in the vents, I don't panic.

That said the straps on my helmet are nice and salty. They definitely need a rinse now and then. Speaking of which, I really should give them a rinse. They are overdue.