The Master worked out how to regenerate extra times, but he had to do something really bad (I don't remember what) but I think it was something that the doctor would never do. So BBC is gonna have to think of something new.
EDIT: I found two paragraphs on his extra regenerations in the Dr. Who wiki:

Susan Foreman triggered an explosion when the Master attempted to kidnap her. He exhausted his regenerative cycle recovering from the injuries sustained in the explosion on the planet Tersurus (EDA: Legacy of the Daleks), where Chancellor Goth found the Master, in a wasted condition. The Master had by this time reached the end of his regeneration cycle and had turned into an animate though rotting corpse. The Master, with the collaboration of Goth, lured the Doctor to Gallifrey in order to have him framed for the assassination of the Lord President. The Master nearly succeeded in acquiring the Eye of Harmony and using its energy reserves to renew himself. (DW: The Deadly Assassin) On the planet Traken, the centre of the Traken Union, still in his degenerated form, the Master plotted to take over the Source, the power behind the Traken Union. (DW: The Keeper of Traken)
And this one:

The Master did manage to steal the body of Tremas, the father of the Doctor's future companion, Nyssa. (DW: The Keeper of Traken) He immediately set out on a new career of villainy in his new body. (DW: Logopolis) Eventually, he found himself taken over himself on the Cheetah World by a foreign influence and began to lose control. He ended up trapped there as it began to die. (DW: Survival)