Excerpted from The Noble Eightfold Path:
Right Speech (samma vaca)
Abstaining from false speech (musavada veramani)
Herein someone avoids false speech and abstains from it. He speaks the truth, is devoted to truth, reliable, worthy of confidence, not a deceiver of people. Being at a meeting, or amongst people, or in the midst of his relatives, or in a society, or in the king's court, and called upon and asked as witness to tell what he knows, he answers, if he knows nothing: “I know nothing,” and if he knows, he answers: “I know”; if he has seen nothing, he answers: “I have seen nothing,” and if he has seen, he answers: “I have seen.” Thus he never knowingly speaks a lie, either for the sake of his own advantage, or for the sake of another person's advantage, or for the sake of any advantage whatsoever.Abstaining from slanderous speech (pisunaya vacaya veramani)
He avoids slanderous speech and abstains from it. What he has heard here he does not repeat there, so as to cause dissension there; and what he has heard there he does not repeat here, so as to cause dissension here. Thus he unites those that are divided; and those that are united he encourages. Concord gladdens him, he delights and rejoices in concord; and it is concord that he spreads by his words.
Abstaining from harsh speech (pharusaya vacaya veramani).
He avoids harsh language and abstains from it. He speaks such words as are gentle, soothing to the ear, loving, such words as go to the heart, and are courteous, friendly, and agreeable to many.
Online, where our speech makes us who we are, these are even more important. If only I had the will.
Lots of Buddhists who have checked out Nonviolent Communication find it full of excellent theory & practice on right speech. In particular, when you don't "have the will" to be honest without being harsh with others, the suggestion is that you practice NVC with yourself rather than to try to force or fake it with others. This usually turns things around pretty fast.
http://www.cnvc.org/main.htm