We stop guarding the US/Mexican border immediately.  Any Mexican who wants to come to America may do so, no questions asked.  If the number of Mexicans living in the US exceeds the number living in Mexico, then we get Mexico.

Effectively, the Mexican people will have voted with their feet, deciding they want to be Americans, not Mexicans.  As we are a country that believes in majority rule, if most of them want it, we give it to all of them.  Boom, instant citizenship for everybody, and land annexation for us.  At which point they start earning minimum wage and paying taxes.  And we get their cacti, tequila factories, and offshore oil fields.