Anybody reading this: Feel free to let me know if any of this is wrong; this is based on pretty thorough research, but I am NOT a conservation expert.
Three things:
Cels love air.
Cels hate light.
Cels hate acid. They even hate themselves the poor things :(
Anyway:
The materials you used in framing are critical:
-Plexiglass must be UV coated and replaced every couple of years. The coating does not last forever.
-Limit light exposure. Even if you have UV protected plexiglass, the lines on the cel will tend to fade in a bright room (I've heard bright fluorescent/incandescent light can also cause fading). Don't keep it in a bright room, or put some sort of curtain on it when you're not looking at it. Light may also increase the outgas process (especially UV)
-Acid: Must be framed with acid-free matting (and tape--professional frame shops have archival grade tape). Museum rag is the best. Not acid-free will cause the cel to approach vinegar syndrome faster.
-Airing out: Air it out every six months to a year. This prevents acetic acid or plasticizer buildup; cels are always outgassing. Check for vinegar smell; if detected, air out and remove ALL matting. Vinegar corrupted matting will continue destroying a cel. When airing, you probably don't have to un-mat the cel; just take the cel in mat and put a fan on it for a bit.
-Temperature: Make sure the wall it's on doesn't change temperature too much (eg, cold in the morning and hot when the sun is on it). Cels like constant room temperature.
-If you really want to be concerned, replace the matting every one to two years, as it absorbs the cel outgassing over time.
-Mounting: The ideal cel mounting setup is here:
http://www.framersworkshop.com/Animationcels/index.html
-Wiggles: Not sure. I'd take the backing off and look at the cel directly; if the cel is being bowed by a botched mounting job, that's bad.
-Backing board should be acid free. Bonus points if you have Microchamber paper in their too (assuming it's not touching the cel). Replace as needed/every 1-2 years.
Note: You can go nuts with conservation both in time and money, and the cel may be fine without all of these. However, If it's a perfect shot of a Ghibli film, you want all the above and maybe more.
I believe Disney/SR labs figured out the best thing is to keep cels in a climate controlled dark room with a weak fan blowing on them. Do keep that in mind if you are thinking real long term conservation; framing is not the best solution for long term conservation.
Source: Worked on framing an expensive cel with a professional framer. I'm not an expert, but I did quite a bit of research. Free free to ask anything else.