Whenever I get a new cel, I always put it into a 
fresh bag, seperate it from its background, and 
put its drawings in a bag as well.  

I buy the sealable cel bags, which was a mistake 
on my part.  I don't know about other series, but 
at least with Ayashi no Ceres cels, keeping them 
sealed causes fading over time.  None of my cels 
got any serious damage, but a couple have a tiny 
part or two that's started to fade.

Therefore I've clipped all of my cel bags to air 
them out.  If you use sealable cel bags, I would 
suggest doing the same.

Just an observation... ^_^;