Put up a wall / fence / SOMETHING at our southern border. Patrol it, and give the patrols some teeth.
Then offer a bounty:
If we were to offer $2,500 to $5,000 a head for illegal immigrants delivered to the Border Patrol by bounty hunters, no questions asked – we don't mind if they turn in each other – with a very stiff hard labor sentence in a chain gang for repeat offenders, it would cost at most a couple of billion dollars a year, and the more it cost the more successful it would be.
There's more at the site. Are these things worth trying?