Sometimes this works…..(updated September 26, 2017)

1. Turn off your Mac (choose Apple > Shut Down).
2. Press the power button while holding down Command-R. The Mac will boot into Recovery mode. When you see the load bar appear you can let go of the keys. It may take a few minutes to load.
3. You will see a macOS Utilities menu in the middle of the screen with four items on it. Ignore it and look at top of screen where you will see a horizontal menu with four items.
4. Choose Utilities > Terminal
5. Enter resetpassword (all one word, lowercase letters) and press Return
6. Select the volume containing the account (normally this will be your Main hard drive)
7. Choose the account to change with Select the User Account
8. Enter a new password and re-enter it into the password fields
9. Enter a new password hint related to the password
10. Click Save
11. A warning will appear that the password has changed, but not the Keychain Password. Click OK
12. Click Apple –> Shut Down
13. Restart the computer as you normally would and you should be able to login

Obviously, with this method someone else could reset your password and steal all your data. There is a way to circumvent this from happening, but if you are the kind of person who forgets passwords the circumvention would probably do nothing more than lock you out of your system forever. Thus, I will not discuss it here.