When you frame a cel, keep in mind two things--
preversation and presentation.  Wether you do it
yourself or have it done by a professional, make
sure only archival materials are used.  I would
recommend using a double layer of mats on top of
the cel to keep it away from the UV glass. I've
only had one piece fade on me and that was due to
not having UV glass.

Make the frame part of the presentation of the 
cel by choosing mat colors that hightlight a 
particular color and draw the eye to cel.  A good
frame should be an enhancement, not a distraction.
A cel on a wall in a plastic box while cool, is
kinda boring--jazz it up with cool colors and a
cool frame. :)  

If you do decide to take it to a professional, 
choose very carefully and DON'T take it to 
some place like Michael's.  If possible, ask a 
local art gallery for a recommendation or call
framers in your area and ask if they have 
experience framing animation art.  If you live in
the San Jose area, I can recommend a great 
framer.  She's done 4 of the 11 pieces I have on
my walls including the one below.  This is a 
Green Arrow model sheet from the 70s Superfriends
cartoon I grew up with.  

Let me know if you have any other questions about
framing cels and good luck! :)

Jen