I bought a piece of 1.5 inch stiff foam to try to fix a sag in a bed. It didn't work but having that thick piece of solid foam around has been a life saver.
Need something flat to put a laptop on? Throw it on the foam. Going to be doing something that requires you to be on your knees for a while? Get the foam!
It went from stupid purchase to something I'd gladly replace if it broke.
I have a set of these designed for musicians, theres an open channel through them, and you put a special "button" at te outward end, that lowers volume without affecting sound quality. I think the company is called "Etymotic Research"