Really Empty the Trash

It’s simple to delete unwanted files using Mac OS X Leopard: Just drag the files onto the Trash icon in the Dock (or select them and type Command-Delete), then choose Empty Trash in the Finder menu (or type Command-Shift-Delete). Alternately, you can double-click on the Trash icon to open it, then click on the Empty button.
Emptying the Trash in this manner clears room on your Mac for other files. But the data hasn’t actually been removed from your computer — the space occupied by these files is simply available to be overwritten by new information. Until this happens, it may be possible for someone to use data recovery software to restore your “deleted” files. And if this information is sensitive or confidential, it could potentially end up in the wrong hands.
That’s why Mac OS X Leopard provides another option for deleting data: the Secure Empty Trash command. Meant for those occasions when you want to permanently and immediately delete files, Secure Empty Trash overwrites your data with digital gibberish, ensuring that your deleted data is gone for good. It may take a few moments longer, but it’s a good choice for deleting data you’re sure you don’t need — and don’t want anyone else to see.

To delete your Trash securely, go to the Finder menu and select Secure Empty Trash. A pop-up will ask you to confirm that you want to permanently erase the items in the Trash. Click OK, and these files will be gone forever.