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. Passwords should not be trivial, predictable or obvious. Obvious passwords include names of persons, pets, relatives, cities, streets, your LogonID, your birth date, car license plate, and so on. Predictable passwords include days of the week, months, or a new password that has only one or two character different from the previous one.