Humidity can be a real enemy to lines too.
But unfortunatly, with some colors, no matter
what you try to do to prevent line fading, the
color will just eat away at it. There seems to be
something about a certain (warm) dark purple that
attacks lines very quickly. Warmer colors tend to
do the most harm. I think that's why so many
lines fade within the characters skin tones. :/
Keeping them in a dry place in a cel book is
probably the best bet.