Password should contain a mix of numericals, special characters, higher case and lower case alphabets.
Let the contents in the password contain structurally confounding symbols like O and 0 i.e alphabet O and number 0
Never have a master password for all of your accounts
Never write down passwords in any of the documents saved in your system
Let the passwords you create can be easily recalled whenever the need arises
Here is a Microsoft website link where you can check what type of passwords are best in terms of security