WHAT IS THIS??

Temporarily seal the engine back up if you can (I usually do this before I knock all the plugs out) and fill the block with vinegar; and let it sit for a day or two. Then knock out all the plugs and flush thoroughly as described above. This helps loosen the sand and all the rust that has accumulated over the years. My driveway smells like pickles when I'm done...
I've heard of people using Draino for this, but it's a little too caustic for me to get soaked with when I flush it out.