Short answer: Keep in cel book in a closed box or 
shelf with no stray light at all. ^_^'

If you mean UV-resistant/UV-proof materials like 
conservation glass or Acrylite, these go about 
98% so there is still that accumulated 2% to 
worry about (if you're so inclined to worry) .. 
But there's also non-UV lightbulbs and UV-bulb 
covers to filter that, too. I've never heard of
other physical methods to preserve the xerographic
lines besides the first 'method' and using 
UV-r materials.

As for restoration, the easiest thing to do is 
to create a separate manual correction layer..