Lots of other readers frequently ask this question. I recommend a two-fold approach. The first, never keep the files on a computer. Keep them on a CD or a USB pen drive. This way, only you will have physical safe-keeping and access to the files. Do not use floppies, as they are highly unreliable.
Keeping these files on your PC, especially when connected to a network or the Internet, makes it easy for anyone to copy them. If storing on a blank CD, create a 'multi-session' CD so you may further add new or modified files.
Second, encrypt and password-protect the files using PGP (Pretty Good Privacy). Get hold of free PGP tools at http://www.gnupg.org or at http://www.cnet.com/downloads.