You pretty much can't. There are various things to help make it less likely to be hacked, but there is absolutely positively no such thing as "unhackable." You should make your password a very cryptic combination of letters, and numbers, as random as possible (there are programs and sites which will generate random strings of letters and numbers for you) and you should make it the maximum length a password can be. Also, check the preferences for various security settings and utilize any that are available to you. Furthermore, don't ever share your password with ANYBODY! I say this because I'm constantly hearing people say that their Facebook is hacked, but then upon investigating the situation further, they tell me that their friend is the one who hacked it, and they share the account with said friend (by the way, this is NOT hacking, this is just blatant teenage stupidity), and if you don't share your account with your friends, they won't be able to go in and change your password as easily.