Diocletian did. He was quite ill, he wanted to hand over the throne to someone stronger in 305 A.D.

The last emperor, Romulus Augustus, was forced to abdicate after Germanic tribes sacked Rome in 476. Historians believe that is the end of the Western Roman Empire.

Although not an emperor, Cincinnatus (b. 519 BC - d. 430 BC) deserves mention for rising to the call of duty and acting as dictator in Rome whilst rival tribes were presenting a threat to Rome. After he finished his task of defeating the enemies, he resigned from his position, despite being in a position of near complete control.

Tiberius (reigned 14-37 AD) didn't abdicate, but he retired to the island of Capri while still keeping the throne. The Praetorian captain Sejanus effectively ruled in Rome until he was caught in a plot to overthrow Tiberius, deposed, and executed.

Let's not forget that of the (according to some sources) dozens of dictators all willingly stepped down after their one- to two-year terms until Julius Caesar, who... well, we all know how that went.

Usually not of their volition.

Later the way the Byzantines and then their successors Turks did it was a mass assassination of all other would-be rivals & successors.

The humane way was to blind your rival then putting him under permanent house arrest. Harem was of course included in the "deal" as well as all the alcohol You-Could-Drink.