I'm sure some of the fancel artists here will
give you tips on pens, etc... but I just had to
say:

DON'T PAINT ON THE CEL ITESELF... PLEASE!!!

I think even the most talented fancel artists 
would be afraid to paint on the cel itself...
one tiny mistake and it's destroyed.  Plus, 
ALL lines fade in time (some just take longer
than others and some are just stored/protected
better)... so that's a hazzard of owning any
production art.  

Kim 
aka Lady Zatara =)