To keep moisture out just use a zip-loc bag.
The technique does work, if you have bright sunlight!
Trying it in the UK in the winter will take an age for example. I believe it's the action of ultra-violet light.
I have also done this with some Zvezda La5FN decals!!
but on a forum I read it the result of a chemical breakdown, and while it can be reversed they will re-yellow in time.
IIRC it was Jennings Helig, and he's involved with making decals. If I can find the info I'll post it here.
Edit - that was surprisingly easy ....
The yellowing is caused by oxidation (redox). Sometimes you can bleach it out with UV light (although in the UK, that might be kind of a dicey proposition for most of the year). The problem is, the yellowing will return. The chemical reaction that caused it in the first place will continue to take place. The organic molecules in the decal film will oxidize, just like if you scrub a steel pan shiny, then leave it out, it will rust again. As long as oxygen is present, the yellowing will return. Even "sealing" a decal on a model doesn't prevent the diffusion of oxygen into the decal.
Full thread here
http://www.britmodeller.com/forums/index.php?showtopic=234919374&hl=yellowing+decalsmaybe the question is how do they take to re-yellow? I'd better see if the decals I did have re-yellowed...now which La-5FN kit was it...and where is it!
HTH
T