When I touch up lines on my cels, I do it right on 
the real cel with an acrylic paint that's 
nonpermanent, so you can't really say the cel has 
been messed up or overly tampered with because you 
could easily wipe the corrective lines off with a 
wet piece of paper towel. I use citadel colour 
black, you can get it at most hobby stores, (or at 
www.games-workshop.com) it's actually for painting 
wargaming miniatures. Nice thing about this stuff 
is you can keep a soft damp cloth on hand as you 
paint, so if you don't like a line you just did, 
you can wipe it right off using no real pressure 
and try again. Keep it in a cel bag or a sleeve, 
and you shouldn't have any trouble with the paint 
staying on, but you won't feel like you have 
indellibly tampered with the cel. Click on the 
link above, and you can see a Tenchi/Ryoko set I 
redid this way about halfway down the page.
Blair
Oh yeah, make sure to use a very fine brush.