[Hobbitt_Fan]

I think that its impressive that in years of using it, his mind was to strong to be corrupted while Sauraman's was. Other than prematurely aging him, it seems the best Sauron could muster was give him false visions of doom or defeat. One of my favorite Tolkien quotes:

Saruman fell under the domination of Sauron and desired his victory, or no longer opposed it, Denethor remained steadfast in his rejection of Sauron, but was made to believe that his victory was inevitable, and so fell into despair. The reasons for this difference were no doubt that in the first place Denethor was a
man of great strength of will, and maintained the integrity of his personality until the final blow of the (apparently) mortal wound of his only surviving son. (Ibid.)