cactusthesaint mentioned something today that I didn't know about:
Congress passed legislation in 1994 stating that presidents elected to office after January 1, 1997 will receive Secret Service protection for 10 years after leaving office. Individuals elected to office prior to January 1, 1997 will continue to receive lifetime protection (Treasury Department Appropriations Act, 1995: Public Law 103-329).
Bill Clinton (elected November 1992) will have secret service protection for the rest of his life, but George W Bush (elected November 2000) will no longer be protected by the Secret Service on January 20, 2019.