I agree that Xerox is probably the easiest way to 
go and it gives a clean line, but I like to line 
trace anyway, so here's what I use.  I started 
with a Sanford Vis-a-Vis transparency marker, 
permanent, non-toxic, extra fine point.  But the 
lines are still fatter than I like it, so I 
switched to a type of ink called arcylic artists 
ink, it's basically liquid acrylic.  It's not in 
pen form, so you have to have a fountain pen or 
something to draw the ink into it.  But it works 
great, for me anyway.  Lines are thin and crisp 
when I used it with a fine tipped fountain pen, 
minus the occasional wigglies from my hand 
shaking.  ^^;  Here's a fancel I made line traced 
with the acrylic ink, my other fancels traced 
with the Sanford pen are here:

http://www.kamidake.com/fancel.html

Hope this helps.  ^^

-ix