Glad the tip was useful! I'm sure I picked it up here on TE last winter when I was desperate for ways to keep my digits warm! :-) And, yes, I DO often stick the foil-wrapped packs into a plastic bag or a small tin with a tight lid. (I should have said that in my first post.) The key is to store them in something air-tight as soon as possible.
Also, if I'm at all dubious about whether I can really get a full second ride out of a pair, I'll use them for walking my dog. (Hour or less.) That way, if the packs don't have a lot of oomph left in them, I'm not stuck part way through a long ride with cold fingers.....