Freecap Disposable Helmet Liners. Box of 8. Use Velcro to hold in place.  These really make a difference in Summer, and throughout the year will keep your helmet clean. 

 

From the manufacturer:

 

the freecap is a hygienic liner of disposable material that keeps your helmet clean, guaranteeing maximum hygiene. Thanks to "velcro" applications it fits inside the helmet until you decide to change it. If you have a second helmet for the passenger, use the freecap will be a gesture certainly appreciated and will allow you to keep the helmet intact and clean .....

 

And, because these are imported from Italy, the company that makes them felt this was a major selling point:: 

 

"Why 'fight with your partner for a hair of different colors found in the helmet?"