Mac OS X has two options to delete the contents of your trash.
The default deletion method just removes the pointers to the file on your drive.
The secure method will actually write over the data, as a result the file is completely gone and can’t be recovered.
To change the default you need to open Terminal (Applications /Utilities) and enter the following commands (in separate lines):
defaults write com.apple.finder EmptyTrashSecurely -bool true
killall Finder
This will set Trash defaults to delete securely and also will change all the menus, and the finder, to Secure Empty Trash.
In the future if you want to change it back to the defaults you can do so by entering the following commands. If you don’t like this change, you can revert it back to the original using the following commands (in separate lines):
defaults write com.apple.finder EmptyTrashSecurely -bool false
killall Finder