Assuming you want to hang them on your walls, UV 
protectant glass is the best way.  You can also 
try hanging them in places that don't get light, 
but that makes them kind of hard to see.  Even 
with the glass, they can still fade though.

If you don't need to display them, you can keep 
them in portfolios/presentation cases/cel books 
in a dark closet.  Some people believe that 
keeping the bag sealed helps reduce fading, 
others think it actually harms!  

Some series/shows just seem to user paint that 
eats the lines faster.  Also, colors like yellow, 
red, and brown tend to increase fading.  There's 
not much we can do about that sadly.

Hope this helps!
Keys