American animators usually use Rapidograph pens 
(the thinnest pen nib).  You can get them at 
your local art store, beware though they are 
about $15 a pop and you gotta buy the ink but 
its' alot better than taking any old black 
marker and drawing on the cel.  

Here is a link to a website that sells them.  
Pay close attention to the little size chart at 
the top of the page so that you can get the 
correct nib size (nib is the part that writes 
just in case you don't know ;)

http://www.rexart.com/kohrapidographs.html

Hope this helps