I love framing my cels! :)  I advise having it
done by a professional framer and make
sure you order all archival materials--
acid-free mats, foam core and UV protection
glass.  When you do hang it, make sure
it's not in a sunbeam or a lot of light--UV
glass can only do so much and if you're not
careful, the lines on the cel will fade.

As far as how much matting, there is no
hard and fast rule, just whatever looks good
to you.  The other two folks who posted 
pics showed two different styles.  The 
Deedlit cel chose to show a lot of matting and
focus  your eye on the image of Deedlit right
in the center--a wonderful choice for head or
torso shots.  The cel of Shiori is wonderfully
done--the matting choice draws the eye to
the image itself.  Good framing should draw
the observers eyes to the image  and 
enhance it, not stand out too much so you're
not looking at the frame and going, "Wow, 
what a cool frame!" :)

Posted below is one of my favorite framed 
cels and I've got more at my gallery.

Let me know if you've got any other cel
framing questions! :)

Jen