Never use a preposition to end a sentence with.
Prepositions Ending a Sentence
The word preposition was coined because such words normally precede the position of their objects in a prepositional phrase. Some people then took this definition to mean that a preposition always had to come before its object and, surely, could never end a sentence.
This "rule" does not always apply when a subordinate clause comes before a preposition. British and Americans agree that one twentieth-century figure who demonstrated excellent command of English in speech and writing was Sir Winston Churchill. Once, when he worked for the Admiralty in World War I, he was rebuked by a superior for putting a preposition at the end of a sentence. He replied by writing back an ironic apology saying that it was "the sort of English up with which I will not put." Of course, that was much more awkward than "something I will not put up with." He made his point.
Some editorial guidelines, especially in England, still call for this "rule," but it is passing. Still, if you think that some people in your audience may be sticklers for this practice, it is better to follow it. Sometimes good will is more important than concise style.