I do not have a cite, but I was of the understanding that if you have two credit cards in your wallet with the RFID chips, simply storing them right next to one another (e.g. in the same pocket) would be enough to stave-off a theoretical RFID theft, operating under the assumption that the two chips in close proximity would provide a confusing and unusable morass of data to any passing-by reading device. I don’t know if any of that was verified, but from the above links it sounds like this is not much of a real problem.