This is actually taken from another forum (IB), and they in turn have taken it from a now taken-down forum (AH), but the advice is really good. I'm hoping to benefit from it by referring to it again and again.
1. Start with "as-salam alaykum" and "bismillah".
This will cool your heart and purify your intention. You will give the person salam (peace), and you will also start in the Name of Allah [swt]; few people can curse someone out after they say bismillah!
2. Begin with praise before criticism.
If you are going to criticize someone, then start out with praise. For example, if you are going to criticize Dr. Zakir Naik for something he said which was wrong, then start with praise, acknowledging all his good work in the field of dawah. This is a part of softening the heart and purifying the intention as well (since you won't post any of this if you are just posting to bash him, instead of with the intention to give naseeha). So this will be like "We recognize and appreciate your tireless efforts in the field of dawah and inter-faith debate."
3. Use a title of respect to refer to the person, such as "brother", or "Ustadh" or "Shaykh", etc, if the latter are appropriate. Add words around this to beautify your speech, such as "esteemed Shaykh" or "my beloved brother", etc. Such titles are MORE IMPORTANT to use when criticizing someone. You can refer to someone as "Anwar" if you are just talking about him normally, but when you are giving criticism of him, then it becomes VERY IMPORTANT to say Imam Anwar. And it would be even better to say the word "HafidhuAllah" after his name...make sure to do all these things *every* time you mention his name!
4. Begin by asking for forgiveness for criticizing, such as "Forgive me for my audacity, dear Shaykh" and the like.
5. Try to reconcile views. This is where I have seen Ustadh Ayman seriously excel at. Two people could be arguing about something, and they are in fact saying stuff that is opposite to each other...but to soften and reconcile the hearts, the Ustadh will reconcile what the two are saying, so that both parties don't think that they were TOTALLY wrong. So it is something like this: "Brother Bassam you are correct in saying this-and-this, but in certain situations such as this-and-that, then brother Fareed is correct in saying that"....and things like "actually you are not saying two opposite things, rather..." etc
6. Close with dua, but never say "may Allah guide you" or anything else which is understood as an insult. It is sad how oftentimes we use duas as insults! If you really wanted Allah [swt] to guide a person, then say that quietly in your own room; no need to announce it to that person's face. Rather, that is done to make it clear to the person that he is misguided! So the intention is not to actually pray to Allah [swt] but to insult another person! So what is better is to include yourself in the dua, saying "may Allah [swt] guide you and I to all that which is right." This will soften the hearts, and ensure that no ill intent was meant!
7. Avoid using nicknames and labels. This is very important, because this is what hurts peoples' feelings the most. So one should not call another person a "defeatist" or a "khariji" ... such labels should be avoided... even when a person is doing something that falls under that category, still avoid it! The Quran says:
"Nor defame nor be sarcastic to each other, nor call each other by (offensive) nicknames: Ill-seeming is a name connoting wickedness, (to be used of one) after he has believed: And those who do not desist are (indeed) doing wrong." (Quran, 49:11)
And nicknames like "o degenerate" and "o wicked one" are not to be used, so what of these worse labels that we invent against each other?
8. Speak in general terms, and do not single out the person, but rather warn the person of those other people, keeping it general. So instead of saying "you are deaf, dumb, and blind", say "O brothers and sisters, remember that the Quran describes the kufaar as 'deaf, dumb, and blind...' so let us not be like that insha-Allah!" Always try to include yourself in whatever you say.
9. Do not post now, but post after 30 minutes. Chances are you will be cooled down after 30 minutes, and won't post the same way had you posted in the heat of the moment.
10. Use smiley faces all over the place. If you don't believe in smiley faces, then you can write out the word "smile" as Ustadh Ayman does. *smile* Smiling softens the hearts, and so it should be done in its e-form, insha-Allah.
11. End with praise of the person and ask for forgiveness. So this is the sandwich approach, whereby your criticism is packaged between praise.