Don't use common passwords
Every time a database of usernames and passwords gets hacked and is published I can't help but look at the passwords that are being used the most. And every time I see the same easy-to-guess passwords make their appearance, again and again ("123456" anyone?)
Therefore, take a look at the list of 500 most common passwords linked below, and if your password is in it I advise you strongly to change it right now.
Also note that "clever tricks" like substituting "3" for "e" are well-known to hackers, and so are patterns on keyboards. If you want to be secure, use random password generators and lengths of at least 15 characters, a password manager, and a unique password for each site.