(In Cullavagga - Cv.IX.5.2)
Someone who is about to admonish another must realize within himself five qualities before doing so thus:
"In due season (Timely) will I speak, not out of season.
In truth I will speak, not in falsehood.
Gently will I speak, not harshly.
To his profit will I speak, not to his loss.
With kindly intent will I speak, not in anger."
Source: In Vinaya Pitaka translated by F S Woodward listed in http://www.beliefnet.com
Five qualities to establish in oneself before leveling a charge:
1) "I will speak at the right time, not at the wrong time."
[C: "the right time" = one on one; "the wrong time" = e.g., in the midst of the Community, midst of a group, in a lottery hall, in a conjey hall, in a sitting hall, on an alms path, when supporters are giving an invitation to request requisites.]
2) "I will say what is factual, not what is not factual."
3) "I will speak gently, and not harshly."
4) "I will say what is connected with the goal (or: the matter at hand), not what is unconnected to the goal (the matter at hand)."
5) "I will speak with an attitude of good will, and not with inner aversion."
Source: In Cullavagga (Cv.IX.5.2) translated by Thanissaro Bhikkhu listed in Buddhist Monastic Code II