If you're looking to frame it the best way 
possible (ie: archival), take it to a 
professional framer, who's used to caring for art 
rather than a high school kid at a craft shop or 
something. ^_^ The framing costs will probably be 
high, but if it's a special cel, or something you 
want the best care for, it's worth it.

If it's an inexpensive cel, or doesn't justify 
all the fuss of a professional gallery, it's just 
fine to do-it-yourself. Just make sure you use 
acid-free matting to help separate the cel from 
the glass (you can get those pre-cut). Glass with 
special coating to help filter UV rays is another 
nice consideration. I like to use acid-free photo 
corners on my backing, and slip the cel into it 
so there's no adhesive or anything on the cel... 
others might have their own techniques.

Where you hang it is going to be an even bigger 
consideration, though, than the materials you use 
to frame it. It's common sense not to hang things 
in direct sunlight, or in a bright room. It's 
common sense not to hang it in a place that 
undergoes major temperature/humidity changes 
frequently, or where there are particles in the 
air... like you don't hang it in the bathroom or 
the kitchen. :o) But even though it seems 
obvious, some people still do it, and then wonder 
why they ruined their art. ^_^;;

You might search the archives for past 
discussions on framing, matting, glass, and so 
on. Good luck!