There are already very nice answers about paper on this and the rpg section, but playing cards are a little bit different and at the same time more specific as they have a special finish and a certain type of paper used.
There are numerous recommendations about roughing up the surface and then using tea bags (for an aged brown look and to oxidate the ink, this sometimes has the sideffect of a teasmell though which of course should be avoided for realism), scratching up the surface and using oil (simulating the play history, as in handoil and nailscratches) and applying coffee to the edges (as they are more exposed there is more wear on these parts),
but how do you get the best results now if you want to make your playing cards seem as if they have been regularly used for a long long time (but this, in a preferably quite short time)?