Protect it with a strong passphrase.
I recommend reading this XKCD comic for how to come up with strong passphrases.
Nice implementation of xkcd passwords.
Security is not terribly hard, but it is completely and totally unforgiving. If you slip up on security, you will suffer, or others will suffer because of you.
Don't slip up on security.
It uses gpg to encrypt each site's username and a random password (using /dev/urandom or CryptGenRandom as the entropy source) in a separate file ~/Documents/Passwords/<domain>.gpg . At the time I first wrote it, I wanted something that was future-proof and where I could decrypt my passwords anywhere, and where a hardware fault while adding a new password was unlikely to cause corruption of pre-existing passwords.
Make your xkcd wordlist (http://google.com/search?q=oxford+3000 is a good start), one line per word, and bzip2 compress it as ~/Documents/Passwords/wordlist.txt.bz2 (or My Documents\Passwords\wordlist.txt.bz2 on Windows) and run the password generator with --alphabet 100 (restricting choices of "alphabets" to those with over 99 characters) and you'll get xkcd-style pass phrases. I also put some numbers in my word list, so I get passwords like
blood Evil pint Urge occupy short railway 38
Diagram elbow Arrest 80 Ceremony monday salt sector
Have a look at my script. It's all of 200 lines. Easy to audit, and I'm not doing any of my own crypto or random number generation.