From Wikipedia:
The United States requires that an individual go in person to a U.S. embassy or consulate outside the U.S. and sign before a consular officer an oath or affirmation that he intends to renounce his citizenship, although exceptions are legally permitted in times of war and under special circumstances.[21][22] During the expatriation procedure, the individual must complete several documents and demonstrate in an interview with a consular officer that the renunciation is voluntary and intentional.
This is great news. I shall renounce my US citizenship in Tijuana. Then, sneak back across my border and live in the shadows.