User passwords are one of the foundations of security. If you use a password that is simple or easy to guess, you're just asking for someone to break into your computer or user account.
Mac OS X includes a password assistant that automatically generates random passwords according a specified level of complexity; it also checks the complexity of passwords that you create. Make use of this feature whenever you need to generate a password -- for Web sites or other services as well as for your Mac OS X user account.
To get to the password assistant, go to the Accounts pane in System Preferences, select a user account, click the Change Password option, and then click the button with a key icon next to the New Password field.
The password assistant can create secure passwords. Click to view larger image.
Even the most complex password can be cracked, however, so remember to change your password frequently. If you don't trust yourself to remember, try setting a monthly reminder in iCal.