A strong password contains at least 10 characters and a random combination of letters, numbers and any special characters that you can type directly from the keyboard; however, avoid the scandinavian åäö and all diacritical marks. Names, words and sequential numbers should never be used.
As an example, yor can think of a song, a poem or some phrase. Then pick the first (or second or ...) character in each word, CAPITALIZE some of the characters and mix with some numbers and/or special characters. E.g. applying that method on the expression "Ask not for whom the bell tolls, it tolls for thee" you could generate the password An4wtbt_itft - this is a very strong password but still easy to remember, only for you!