How to reset password for Mac OS X 10.7 Lion

• Shutdown Mac
• Hold Command and R keys and Power on Mac
• Keep holding keys until you reach the recovery screen
• Select Utilities -> Terminal
• At the terminal, type: resetpassword
• Press enter
• The Reset Password Utility will be started
• Select the hard drive
• Select the user account
• Enter a new password for the account

Here is a nice video that I made showing you how this can be done: