If it's a smaller deck and they can be shuffled, that'll straighten 'em out. It may take awhile but they will flatten out.
If it's the large version that may be a problem. You might make some cardboard inserts that will fill up the excess space in the box so the cards are always stored flat. Take them out once in awhile if you're not using them and flex the whole deck back and forth to try and work the curve out. I've had pretty good luck doing this but it takes a very long time. I'd rather go slow though, it's easy to mess up a deck by bending and twisting it too much.
Something I did one time was take two books and space them apart so the ends of the deck rested on the edges of the books. Then I put something heavy on the deck so it bent slightly in the opposite direction of warp, then let it sit overnight. It wasn't a miracle cure but every bit helps.
The trick is to loosen the cards up so they lay flat. If you can shuffle 'em you can accomplish this relatively quickly, otherwise you just have to keep working the cards and store them flat.