I have heard that modulating by diminished fifth/tritone is the hardest of all modulations. Interestingly my non-musician husband has just composed a tune with a smooth tritone modulation (ie. from C to F sharp below). Apparently the key to his success was: the last chord of his tune before modulation happened to be G major, which continued to F# major (tonic of the new key). In other words, root movement by a semitone/minor second.
Does anyone have other strategies for a successful tritone modulation, especially if the tune does not lead to such a "semitone/minor second" chord progression?