Let the platform that a candidate runs on be a contract with the people. When he is elected, if he doesn't do what he said in his platform contract, execution. If he is literally stopped by the opposition from a timely follow-through, so it isn't entirely his fault, life in prison immediately.

When the next person in line takes over after the first guy has been executed/imprisoned, he has a choice. Fulfill the platform contract of the first guy, be executed/imprisoned like the first guy... except if he voluntarily immediately resigns without taking office.