Mix numbers with capital and lowercase letters. Most people have passwords starting with a capital and ending with numbers - avoid that trap.
The longer the password, the harder it is for someone to guess it. Try using a word at least eight letters long.
Use a pass 'phrase' rather than a pass 'word' e.g. ILikeBlueCh33se.
Don't use the example above, or keyboard patterns (asdf) or sequential numbers (1234).
Don't use a password that contains personal information (name, birth date, etc).
Don't use well known acronyms (e.g. colours of the rainbow: ROYGBIV) or words that can be found in a dictionary.
Never tell your password to anyone, even someone from British Airways.
Never write your password down or send it via email.
Change your password often for better security.