If you want to keep the Mexicans out of the U.S. then instead of putting up fences just line the border with soap. Ground up Ivory or liquified Dove...it doesn't matter. Just spread soap all over the border and it will keep 99.9% of the Mexicans out of this country.
If that doesn't work then put up a taco stand every 100 yards advertising free tacos. Put mild sedatives in the tacos and that will entrap roughly 85% of them.
How about large billboards saying, "Not Hiring". Will that work?
Sharks and Crocs in the Rio Grande...that would be awesome as well!
Damn, I should work for the the INS!