If, in addition to personal reproof, any other option exists that could preclude the necessity to speak negatively of someone, it must be pursued. Negative speech, even for a constructive purpose, is lowly if it can be avoided.
In the same vein,when speaking negatively is necessary, one must carefully calculate just how much must be related in order to achieve the desired result. To relate any unnecessary negative information would be to speak loshon hora.
Furthermore, even if it is clear that someone must be made aware of certain derogatory information about another person, one should choose the least blatant means by which to communicate that information. Directing someone to where he will become aware of information on his own is preferable to expressing it verbally.
In comparing two candidates for a position, stressing the strengths of one is often sufficient without actually articulating what the other lacks. In all cases, the least negative course is the one to follow.
Sefer Chofetz Chaim, 16 Cheshvan, page 140