If you're wondering about the credit card-bag trick, here's a basic explanation: After extended use, the oxide (written information) will "smear"and produce lower-level mag fields. These will give a false value and effectively not be approved by the checksum. By increasing the distance (via bag, tape, etc) between the read head and the strip, the sensitivity falls and only the original data (higher-level mag fields) will be read.
http://www.newscientist.com/article/mg19225752.500-money-bag...