Guidelines For Choosing Password

taken from the manual page of passwd by Cristian Gafton <gafton@redhat.com>

Remember the following two principles:
    Protect your password.
        Don't write down your password - memorize  it.   In
        particular,  don't  write it down and leave it any-
        where, and don't place it in an  unencrypted  file!
        Use  unrelated  passwords for systems controlled by
        different organizations.  Don't give or share  your
        password,  in  particular to someone claiming to be
        from computer support or a vendor.  Don't let  any-
        one  watch  you  enter  your password.  Don't enter
        your password to a computer you don't trust  or  if
        things  Use  the  password  for  a limited time and
        change it periodically.
    Choose a hard-to-guess password.
        passwd will try to  prevent  you  from  choosing  a
        really bad password, but it isn't foolproof; create
        your password wisely.  Don't  use  something  you'd
        find  in  a dictionary (in any language or jargon).
        Don't use a name (including that of a spouse,  par-
        ent,  child, pet, fantasy character, famous person,
        and location) or any variation of your personal  or
        account  name.   Don't  use  accessible information
        about you  (such  as  your  phone  number,  license
        plate,  or social security number) or your environ-
        ment.  Don't use a birthday  or  a  simple  pattern
        (such  as  backwards,  followed by a digit, or pre-
        ceded by a digit. Instead, use a mixture  of  upper
        and  lower case letters, as well as digits or punc-
        tuation.  When choosing a new password,  make  sure
        it's  unrelated  to any previous password. Use long
        passwords (say 8 characters long).  You might use a
        word  pair  with punctuation inserted, a passphrase
        (an understandable sequence of words), or the first
        letter of each word in a passphrase.

    These principles are partially enforced by the system, but
    only partly so.  Vigilence on your part will make the sys-
    tem much more secure.