If you’re looking for something that’s even more secure than the Secure Empty Trash feature for MacOSX, Permanent Eraser is your answer.
It’s open source, and freeware, and it implements the Gutmann Method that rewrites over the data thirty five times, rather than the usual DoD seven times method. Now don’t get me wrong here… this is pretty secure to a point. But you can still recreate data via forensic means unless it’s been totally demolished. While the Gutmann Method does a pretty darn good job of that, that doesn’t mean it’s flawless like everything else. But it definitely would be very secure for very sensitive data such as your credit card numbers and passwords. Why you’d be storing that in a file? I don’t know.