I have in the past used a wallet with a cord attached to the inside pocket on my suit. That way if someone does lift it they are still attached to me. I started doing this after someone tried to steal my fathers phone in Borough Market without realising that it was attached to him by a cord. The guy snached the phone from my fathers belt, legged it and then looked very suprised when it was snached from his fingers by the cord attached to my fathers belt.