Never use the same password twice. In fact, good access control systems prevent you from choosing a new password that is similar to your old one. When you are selecting a new password, choose one that is quite different from your previous password. Passwords should be changed frequently. The shorter the life of a password, the better it is.
Some systems force users to change their password at predetermined intervals.
Passwords should be at least four characters in length. If the system allows a password longer than four characters, then it is recommended that you use a minimum of six characters. Longer passwords are harder for others to guess. Passwords should contain a combination of alphabetic, numeric and special characters. Avoid using any dictionary words.