Make sure that every user on the system chooses good passwords. Where possible, have the system enforce good password selection. Look into tools like cracklib to do dictionary attacks on perspective passwords and reject insecure passwords. A good password should be long, as least eight characters. Where possible, allow users to use pass phrases instead of passwords.
Pass phrases tend to be easier to remember and harder to steal.
Make sure that each password or pass phrase has multiple types of characters. Ideally every password should have lower case letters, upper case letters, numbers, and punctuation. Make sure that users change passwords frequently. Passwords should be changed at least every six months, every two months for privileged accounts. Routinely use a password cracking program like Crack, qcrack, or l0phtcrack to check your password security.