Grand Theft Auto 5 guru Typical Gamer put together a great video showcasing the newly discovered Invincibility cheat. The video offers some suggestions on how to maintain invincibility beyond the 5 minute limit set by Rockstar.
Invincibility Cheat Code
PS3: Right, X, Right, Left, Right, R1, Right, Left, X, Triangle
Xbox: Right, A, Right, Left, Right, RB, Right, Left, A, Y