I've been playing Magic: the Gathering for a while now, and I have collected a fair amount of the "penny" card sleeves (clear plastic, no frills). Now they're old and, due to humidity or some other factor, they have a tendency to stick together. Is there a way to store them and/or play with them to fix/prevent this?
I'd say if your deck is even relatively valuable, I'd upgrade from the penny-sleeve treatment. Penny sleeves are fine for protecting bulk rares and draft decks but the premium stuff is usually a tighter, tougher sleeve with opaque backs (which matters if you want to do any serious competitive play). I would avoid Dragonshield Gloss though because the backs get scratched pretty easily from the pointy corners while shuffling. The Mattes are fine and actually real good.
On the topic of cleaning and storage, here's a few suggestions. if you live in a very humid area, throwing one of those silica packets in your deck box can do quite a bit of work. To avoid collecting those weird little dirt clumps, using a play mat removes the dirty table element and will prolong their cleanliness for a bit (and plus they're nice to play on). If they do get dirty, rubbing alcohol will clean them up decently but after awhile, just replace them.
If you're just looking for new penny sleeves and the like you might want to check you LGS. For example my local store has a box of free sleeves where you can drop of old sleeves or pick up some sleeves for free. While they aren't in the best condition I've sleeved many a commander deck with these free sleeves. If you're lucky you might even find a full set of dragonshields or something like I have.