My law practice is stored in an encrypted disk image. Encrypting your practice files is like locking your files in a bank vault.
How does it work? A disk image is like a directory but when it's opened it requires a password. Once opened the directory will be available until you eject it (like a CD).
A nice benefit of using a disk image is that you can back up your computer without worrying about securing the backups (something you should worry about!). You could use a USB key to back up your practice or an online service like BackBlaze.
Here's how you can make an AES 256 encrypted disk image on a Mac: support.apple.com/kb/ht1578.
There's a popular open-source program for Windows/Linux/OS X called Truecrypt that can be used in the same way.