Many electronics come with silica gel packs. Save em and use them for anything you want to keep "dry" Go store hopping Wal-Mart, Meijer, any place that sells LARGE electronics and ask if they have anything that has been un-boxed that day... or ask them to save some for ya.
There is this white crystal "kitty litter" that I am not sure is marketed any more. If you see some buy a box. Fill up some pantyhose socks and you have made your own silica gel pack! This may not actually be silica gel but this stuff is supposed to suck up kitty pee. I have use it with great success.
or if you have some money to burn:
http://www.silicapack.com/products.htm
(this site also has instructions on how to recharge silica gel)
The key as others have said it to have an air-tight / water-tight container.
Tupperware is not good enough for long term.
Something with a rubber / silicone gasket.
I use an ammo case filled with plastic bottles that also have gaskets with a couple of my home made whit crystal kitty litter packs.
I don't use this stuff in the cat box because it is translucent and I think that would be nasty)