Simple. Create circumstances for Congress to overturn the 22nd Amendment.
The key is overturn - as in, it has to have been passed to begin with.
edit:
Background: The 22nd Amendment to the U. S. Constitution was passed in 1947 to limit all future Presidents of the United States to two terms, and to limit any President who succeeded via the Vice Presidency to one four-year term if more than two years served under term of another president. It affected Dwight Eisenhower, Richard Nixon (sort of), Ronald Reagan, Bill Clinton, George W. Bush and now Barack Obama, and excluded incumbent Harry S Truman.