At the end of Frank Miller's The Dark Knight Returns, Batman beats up Superman and delivers the iconic quote:
I want you to remember, Clark. In all the years to come, in your most private moments, I want you to remember, my hand, at your throat, I want you to remember, the one man who beat you
Then Superman notices that Batman's heart was growing weaker and before he could say or do anything, Batman collapses and dies (apparently).
So, how exactly did he fake his death? Superman obviously could hear his heartbeat and he himself declared Batman dead.
Did Batman use some sort of serum to bring his heartbeat to a unbelievably low beat that even Superman couldn't detect?