Magicians' Fanning Powder!
For everyone who has a sticky or clumpy deck that won't spread or shuffle easily – try magicians' fanning powder. It makes a HUGE difference on most decks of this nature. It's usually found in magic stores and it's also sold on Amazon. It's not expensive, a little goes a long way, and it seems to be long-lasting.
For example, I couldn't get the Mary-el cards to separate when I fanned them out because they were sticking together in clumps. After applying fanning powder, they spread easily and effortlessly. I went from hate to love.
Use very little of the stuff. I was applying it all wrong at first by sprinkling it directly on the cards. That's WAY too much. Sprinkle a little on your fingers or palms of your hands, rub your hands together, then lightly rub your hands across the front and back surface of each card. You should be using so little that you can't see any powder on the cards (even on dark cards). Try it first on a few cards and use more if needed.
It has worked wonders on every deck I've applied it to with the exception of one, the PCS Commemorative. The deck wasn't all that sticky to begin with but I wanted to see if I could improve it a little. Didn't help and made it worse, so I cleaned the powder off. Overall though, I have made a good number of sticky, clumpy, and old decks much more useable.