I think that Bilbo, Frodo, and Sam alone of all beings ultimately escaped the Ring's power because they all willed to surrender it. Bilbo chose to pass it along, and though he wavered when it came down to it and had to be helped (a couple of ways) by Gandalf, he managed it - as Gandalf himself points out. Frodo willed to take the Ring to Mount Doom, and even though he was terribly weakened and faltered at the last moment, a way was made for him to triumph after all. I also agree that Sam's brief time of holding it, and the noble reason he took it in the first place, and his love for his master, all combined to give the Ring very little hold on him. He surrendered it most easily.