online: Eraser Home Page
online: Peter Gutmann
If there's one person who truly understands the science of secure data deletion, it's Peter Gutmann, computer scientist, at the University of Auckland, Auckland, New Zealand. He developed a method back in the mid-nineties for securely wiping data. It overwrites disk files with a formidable thirty-five passes!
Much has been made of Peter Gutmann's wipe. Like so may others, you may choose it for your purposes. However, in the epilogue to his famous paper online: Secure Deletion of Data from Magnetic and Solid-State Memory, he notes that the thirty-five wipe technique provides coverage for all types of drives and that no single drive requires all thirty-five passes. Furthermore, for any modern PRML/EPRML drive, a few passes of random scrubbing is the best you can do. Gutmann's wipe is very time consuming. It slows the process of encrypting and sending. He recommends a few random passes instead. I like the DOD wipe. Seven passes is good. Take a look at the Epilogue in the online document linked above.
There's more you can do--if you're so inclined
While browsing Gutman's paper, don't miss the Further Epilogue. It provides info on making wiped files even more obscure. Essentially, he suggests you wipe all windows free space frequently. His point is: a wiped file gives the searcher a place to focus--the wiped file. When you wipe all free space, you wiped file disappears into a sea of wiped data. It no longer stands out.
This is excellent advise, if your security requirements demand such extremes. It can be accomplished by downloading the full Eraser package and using it's capabilities to wipe windows free space, pagefile, etc. This is a very good thing.