A strong password maybe??
I take you are referring to people hacking your password right? In which case you can't stop someone from guessing your password, if they have enough time time they'll get it. You can make that a lot harder but using a long password made up of letters, numbers and psecial characters.
A strong password is good. Don't use the same password for all your accounts. Update your password periodically.
However the strength of your password makes little difference if it is stolen. Make sure you're connected to facebook though HTTPS, an encrypted connection. The location bar should display a padlock symbol. This helps to prevent man in the middle attacks.
Avoid signing into accounts from Internet cafés or other public networks. Computers infected with malware such as key loggers can compromise your password. To reduce the risk, type an incorrect password then wipe over with the mouse cursor to correct. Don't attempt to correct it with the backspace or delete key as these key presses can also be monitored.