DO the following:

DO change your password regularly.
DO pick a password you will remember so you DON'T have to write it down.
DO use a mix of uppercase and lowercase characters.
DO use punctuation marks and special characters such as #, $, %.
DO choose a line or two from a song or poem and use the first letter of each word, preceded or followed by a digit. (e.g "Do you know the way to San Jose?" becomes the password DYKtwTSJ?).

DON'T do the following:

DON'T make obvious choices like your last name, first name, nickname, birthdate, spouse name, pet name, make/model of car, or favorite expression.
DON'T choose your username as your password.
DON'T share your password with anyone.
DON'T use blank spaces in your password.
DON'T use a word contained in English or foreign language dictionaries, spelling lists or commonly digitized texts such as the Bible or an encyclopedia.
DON'T use an alphabet sequence (lmnopqrst), a number sequence (12345678) or a keyboard sequence (qwertyuop).