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.