[url=http://esl.about.com/library/grammar/blgr_prep8.htm] About.com[/url] Adjectives that take a construct of <~ of someone to do> :nice / kind / good / generous / mean / stupid / silly / intelligent / clever /sensible / impolite / rude / unreasonable (EX)
I agree with Clive. "It was sensible of him to ..." says "He was sensible to ...". "It was sensible for him ..." says "(I think that) his staying away from the bag was a sensible thing to do".
In the first case the emphasis on the fact that he is sensible; in the second, on the fact that staying away from the bag is sensible.