The first well-known incident of a leader extending his term indefinitely was Roman dictator Julius Caesar, who made himself "Dictator for life" in 45 BC. His actions would later be mimicked by the French leader Napoleon Bonaparte who was appointed "First Consul for life" in 1799.
Since then, many dictators, especially those in post-colonial Africa, have adopted similar titles.
Ironically, most leaders who proclaim themselves President for Life do not in fact successfully serve a life term. Most are deposed long before their death although some, such as François Duvalier, have managed to run out the clock.
List of Leaders who became President for Life (with date of proclamation)
De facto Presidents for Life