Erase free space!
"Secure Empty Trash" is a great feature for confidential data, but if you erased a file and forgot to do it, it is still possible to truly erase it. In "Disk Utility", under "Erase", there is a feature called "Erase Free Space...". It will actually overwrite all the free space on your hard drive. You can choose "Zero Out", "7-Pass" or "35-Pass". Zero Out is usually fine, but if you want to make the data even harder to recover, 7-Pass is also a good option. 35-Pass takes just way too much time!