The other half of illegal immigration is losing track of people who overstay their temporary visas (visitor, student, H1B, etc.)
We need a 21st century way for temporary visa holders to periodically check in with Homeland Security and ensure they leave on time.
I would think in this age of Computerized Everything, there would be a system that logged each visa holder as they left. Those who remain after their visas have expired should automatically have a bench warrant issued for their arrest and deportation.