Forgot Your Account Password
For Snow Leopard and earlier
For Lion/Mountain Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
When the menubar appears select Terminal from the Utilities menu.
Enter resetpassword at the prompt and press RETURN. Follow
instructions in the dialog window that will appear.
Or see Reset a Mac OS X 10.7 Lion Password and
If you're running Mountain Lion, you can boot into your Recovery partition by holding down the Command and R keys whilst booting. When in recovery mode, select Terminal from the Utilities menu. In Terminal, type:
...a small app will open allowing you to change teh password for a chosen user. Select the admin account, enter the new password twice, give yourself good hint, and click OK and then restart. Upon restarting you'll be able to use your 'new' password.