protecting the cord, and replacing head-phone covers
The headphones I'm talking about are the "on the ear", not "over the ear" style. Sony MDR-G45s, to be precise. Nothing fancy.
There are two parts of a set of headphones that seem most prone to damage with age: the cord, and the foam ear-pads. Both seem to be affected by prolonged contact with whatever chemicals are found on my skin. The plastic insulating layer on the cord gets brittle, and the foam pads get fragile, and eventually sticky.
In an effort to prolong the life of my latest set, I ran the cord through a fabric sleeve. (It was actually the front hem of an old shirt, with the buttons cut off.) So far, so good. As for the pads, I cut a circle of fabric about twice the diameter of the speaker unit, ran a row of zig-zag stitches around the circumference to prevent unraveling, then hand-sewed a long running stitch just inside the zig-zag circle. When I pulled the running stitches tight, it formed a sort of pouch, with the driver inside. You might be reminded of a tiny shower-cap, if you see it. So far, I haven't felt the need to put padding inside the cover, but that would be easily-enough done with a few fat rings of fabric sewn together and slipped inside the cover.
I just can't stand the thought of perfectly good headphone drivers going into the trash, because a few cents worth of foam went bad!