The following guidelines will help you to create a strong password:
Make the Password at Least Eight Characters Long — The longer the password, the better. If using MD5 passwords, it should be 15 characters or longer. With DES passwords, use the maximum length (eight characters).
Mix Upper and Lower Case Letters — Red Hat Enterprise Linux is case sensitive, so mix cases to enhance the strength of the password.
Mix Letters and Numbers — Adding numbers to passwords, especially when added to the middle (not just at the beginning or the end), can enhance password strength.
Include Non-Alphanumeric Characters — Special characters such as &, $, and > can greatly improve the strength of a password (this is not possible if using DES passwords).
Pick a Password You Can Remember — The best password in the world does little good if you cannot remember it; use acronyms or other mnemonic devices to aid in memorizing passwords.