While psychology would affect it somewhat, I'm sure climate also plays a role. As mentioned above, practicing with an old deck will help. Another tip I can give you is to wash your hands with some sort of soap that will dry them before sitting down, and when you need a slight tackiness, grip a cup of water or soda which has condensed a little so your hands are in shape.
That ought to do the trick, if it can fit into your context (and I can't imagine one where you can't have a drink).
To be honest, though, the idea of using older, clumpier decks is probably the best. Poker players aren't magicians. Depending on your setting, the cards WILL get clumpy fast, what with all of them eating, not washing their hands every time they touch cards, etc. Especially if they're plastic, those cards are likely to be in play for a long time, since they last a couple of months. With a paper deck, it'll take a few hours before you're reduced to that state anyway.
Getting used to old decks is very good advice, and I was actually going to throw that idea in, but you beat me to it! Most of real poker games with friends will have old decks that have been used numerous amounts of times. Get used to different condition decks by practicing with decks on this basic scale, 5 being completely worn in and crap, and 1 being new condition:
5 - Complete crap deck that would make you cringe, and cards stick and clump a lot
4 - Slightly better condition but still is crap and cards stick, clumpy
3 - Middle condition, worn in pretty good, cards lightly stick and clump
2 - Breaking in stage, cards are not new but aren't worn in yet
1 - Brand new, out of box
Practice going from 5 to 1, adjusting to different feels of the cards and the different ways they work with your hands. Also, try and judge a deck based on this scale by its looks, and if you're accurate, you can make the adjustments in no time to suit you properly.
Also, going from 5 to 1 is always the best, because the deck slightly gets better as you go through the scale. Getting used to the crappiest decks makes adjusting to a newer deck a lot easier.