Well, the truth of the matter is that these 
anime cels werent made to last longer than it 
took to snap them with a camera.  After that, 
the studios dont care.  They're mass produced, 
half of the time the people who paint them are 
students, interns, or even just people in temp 
agencies.  I dont think you can prevent it any 
other way than how you normally would.  Keep the 
cels in a dark place, room temperature, not too 
dry, and definately not too moiste.  Also, I 
guess you can poke holes in the cel bags too, to 
vent them a bit. That really is too bad that 
happened to your cel.  Disney cels are really 
really bad about that, or so I hear.  It is 
fixable, but it costs like a mo-fo.