All of the above-all of these techniques can be used to create a strong password.
mnemonics can be used to make your password easy to remember (e.g. condense a rude phrase or a funny joke into a string of letters, add numbers and symbols, etc.)
a password strategy can be developed to help you remember your passwords e.g. substitute letters for special characters, incorporate elements unique to the system or service into your password, or develop a pattern such as using memorable lyrics from meaningful songs. ensure your password is complex and to some extent random to avoid them being guessed if your password strategy is found out, or one or more of you passwords created using your strategy is compromised.
encrypted password management software can be used to manage your passwords if you have a particularly poor memory, however; be aware that these programs are often targeted by intruders. they are also rendered useless if a weak master password is used to gain access to all other passwords.