I've found that a photograph is the best thing to 
use to try to separate a sketch from a cel. It's 
thin, but laminated enough to be tough, but not 
so tough as to dig into the cel paint. 

First, I use the photo (image side down) to 
explore where the sketch is stuck and whether the 
cel has already been damaged (usually in the hair 
tips).  If I'm lucky, the two will come apart 
during this process.  

If I'm not lucky, I now know what areas are most 
tightly stuck and whether it's worth my while to 
continue.  If the cel has already been damaged by 
a previous attempt to unstick it, you're probably 
better off leaving the sketch on.

But if you're still willing, flip the two over, 
cel side down, and see if you can't use the photo 
(image side toward the cel) to separate the two.  
A little water on the worst spots will, as Xellos 
says, limit the damage to the sketch.  

The important thing is to keep the cel flat, 
since if you bend the cel away from the sketch 
(as I did once), you run the risk of having whole 
chunks of paint stick to the sketch, and then you 
are left with an unstuck but visibly damaged cel.

I'd say it's a risky but often worthwhile effort, 
and I'd encourage you or anyone else to practice 
first on cheaper cels to see what could happen 
before you try surgery on your $500 wishlist 
beauty.