Luckily RFID tag signals can easily be blocked. This means that you will have the option to use the tag whenever you want, and prevent others from being able to read it.
The signal sent out by a RFID tag is easily blocked by metal. This means that placing the RFID tag inside of a Faraday cage will prevent the information from being read.
There are already two Instructables on how to build RFID blocking containers:RFID Secure WalletMake a RFID Shielding Pouch Out of Trash
Or if you would rather spend money on something you could build, head over to Think Geek for their RFID blocking wallet
and RFID blocking Passport Holder