Google provide the following advice for choosing a secure password (from https://www.google.com/accounts/PasswordHelp)
Tips for creating a secure password:
Include punctuation marks and/or numbers.
Mix capital and lowercase letters.
Include similar looking substitutions, such as the number zero for the letter 'O' or '$' for the letter 'S'.
Create a unique acronym.
Include phonetic replacements, such as 'Luv 2 Laf' for 'Love to Laugh'.
Things to avoid:
Don't use a password that is listed as an example of how to pick a good password.
Don't use a password that contains personal information (name, birth date, etc.)
Don't use words or acronyms that can be found in a dictionary.
Don't use keyboard patterns (asdf) or sequential numbers (1234).
Don't make your password all numbers, uppercase letters or lowercase letters.
Don't use repeating characters (aa11).
Tips for keeping your password secure:
Never tell your password to anyone (this includes significant others, roommates, parrots, etc.).
Never write your password down.
Never send your password by email.
Periodically test your current password and change it to a new one.