For strong passwords, make them long, and include a good mix of different characters: letters, numbers and symbols. That's the key takeaway from the New York Times' article on passwords; Guard That Password (and Make Sure It’s Encrypted):
For a pretty strong password, think 10. If your password contains 10 characters, you should be able to sleep well at night — perhaps for 19.24 years.
That’s how long it would take hackers to try every combination of 10 characters, assuming that the password is encrypted and that the hackers have enough computing power to mount a 100-billion-guesses-a-second effort to break the encryption.
A link in the article points to a Gibson's Research Corporation web tool which lets you calculate how long it'd take for a brute force attack to get at your password. Build strong passwords today people!