The xkcd post is pretty good advice, and would lead to much better passwords than what most people currently have, but it's not unbreakable. The biggest risk with that kind of password is dictionary attacks, where instead of guessing alphabets, the software guesses combinations of common words.
The problem with that password is that the words used are extremely commonly used, and thus would be tried in any dictionary attack. The best way to do it is to use 3-4 non-common words. That is a pretty strong password.
Alternatively, you could use a password manager, and have it generate a random 16-character password.
Yes, that's what I do with 1Password, usually going with five words. Thanks!
There is no such things like unbreakable password - so we are not looking for them. We are looking for passwords which will be very hard to break, so to be broken it would take extreme high costs within extreme long period of time. This is the first thing. The second one is, that I am to lazy to do the math, and I do not know how many common words are there in English, but I am guessing that entropy of such password is still higher than this first hard to remember password in cartoon.