I agree, I hate to make up a batch and then end up throwing away most of it. I saved some once forever (6-8 months or something). It looked really nasty, seperated and "skin" on top, but I figured I might as well try to use it before making up a new batch. So I stirred it up and let it sit an hour. The "skin" pieces softened and it was a perfect consistency! I made my flowers and waited for them to harden.
And waited... and waited.
They never really did get crisp. Maybe it was humid or something but they kind of came out rubbery. I gave up and threw the rest away and made a new batch.
Since then I read a thread (sorry, it's saved at home but I'm "working") about mixing up small batches. I don't remember the measurements but it's something like 2 T meringue powder and 1 T water, and beat it up. I love it!!! There were some other ideas as well that some people use to make up tiny batches. I don't bother trying to save it anymore, although I'm intrigued by Snowshoe1's idea of having pre-made colors on hand.