Offer free plane and bus transportation back home to every illegal immigrant in the U.S. Being unemployed in Mexico is a lot cheaper for all concerned than being unemployed in the U.S.

Do it right now. We could call it the:

I'll Be Home for Christmas plan

The government should set up booths at Greyhound stations. All an illegal immigrant has to do is walk in with his luggage, get fingerprinted and photographed, sign a legally binding document that he won't be coming back and will go to jail if he does, and he'll be on his way home on the next bus.