As an afterthought, it is probably not easy to do. However, you may find a utility that will Secure Delete (overwrite a few times) your blank space on the Hard Drive. When you put items in the Trash can and delete them, the area that they used to occupy is marked as being writable again. Whether anything actually gets written there depends on the amount of stuff you install or move around. The OS will try to keep all files and programs in a contiguous area but it will also put parts of files and programs in places that it sees as being free. Secure Delete will mark the area for writing and then Write a series of Random 0's & 1's to that area. I believe it does this a number of times which is why Secure Delete takes longer than the usual delete.
Try this utility for size: http://www.apple.com/downloads/macosx/system_disk_utilities/shreditx.html

hope this helps

P