It's damn near impossible with the current entry/exit control system. Most of it is luck, or like in my office, a good relationship with the airlines. We've approached the outboard airlines and have them fill out an I-94 for every alien leaving the country that does not turn one in. Since we started this system about 9 months ago, we have apprehended dozens of overstayers. We have even had some apprehended in other ports. Unfortunately, the airlines are not required to do it, and it would not work at the land borders.
Thanks for all the great data. I wish I knew how to do formats like those you've copied! I've tried, but just couldn't make them work for me.