First, please remember on the iCarly pages that we are all on the same side. It has got far too contentious around here. The Seddie page has given many examples of what NOT to do on an open forum.
The first thing is to be respectful of everyone else. This means many things.
One facet is that there should be no ad hominem attacks against users, someone who is not as harsh as you are with respect to Jennette, Madisen, and Nathan is therefore not critical. We have all done similar analyses of what they have done and have come to our own conclusions. They may not be yours because we evaluate different actions differently and have different points of view.
Another is that there should be no ad hominem attacks against actors, actresses, etc., such as Madisen Hill by the hardest core Jathaners, Nathan by the super-McCurdians, and Jennette by the mega-disillusioned. I have no background or history on any of this; all I have to base anything on is what I have seen in the last couple of years. And nothing I have seen has justified the most vitriolic accusations laid here. For all we know, any or all of us could be wrong about anything and/or everything.
It also means that you should not make false generalisations, such as claiming that all Arianators want to destroy Jennette McCurdy. There are several here who clearly don't, so don't claim that all Arianators are evil minions of the Grande She-Devil.
Similarly, do not use false dichotomies to speak with others, such as saying that because Matt Bennett is supporting Jennette McCurdy, then he does not get along with Ariana Grande. In this example, one counter would be that if there is going to be a patching up of the relationships, then it must come from people who are OK with both sides.
BTW, false dichotomies and false generalisations are sure ways to garner opposition from those who might remain neutral
The second thing is if you are on a public forum, then keep your cool.
You and other on the forum may be antagonists, but getting into a huff or being accusatory gets you and them nowhere. The people you run into online are not worth getting upset over - including us. Most importantly, rather than typing something you regret, get up and shelve your internet access. As a corollary to this, if you run off in a huff or get into a flame war by flinging ad hominem attacks at all and sundry, it is a sure sign that you have lost control of your head. In a word, don't do it.
Just as I mentioned in my shipping blog, remember that people who disagree with you are not necessarily being uncritical of those they support, overly harsh on those you support, etc. Odds are they have a different point of view from you; to that person, her/his point of view is (emotionally) more valid than yours. Moreover, you have blind spots and biases that may be perfectly justified to you that won't appear to be so to others. This is the reason you must be respectful: with very few exceptions, people believe something that they believe are for very good reasons; they will treat your lack of accommodation of their point of view as a personal attack and hence will show similar lack of respect for your point of view.
The third and hardest part is to be self-aware. If you know your own strengths and weaknesses, take a step back from the keyboard when someone or something upsets you. Figure out why you got upset and try to reply respectfully.
Within self-awareness is that you can't delude yourself. Imagine me as a young Beatles fan hearing the Fab Four say things like these:
"You have to be a bastard to make it, and that's a fact. And The Beatles are the biggest bastards on Earth." --- John Lennon
"Someone said to me, 'But The Beatles were anti-materialistic.' That's a huge myth. John and I literally used to sit down and say, 'Now, let's write a swimming pool.'" --- Paul McCartney
"After having played with other musicians, I don't even think The Beatles were all that good." --- George Harrison
"I got involved with a lot of 'medications' - and you can listen to my records go downhill as the amount of 'medication' went up." --- Ringo Starr
The Beatles were the ultimate group that had an image and sold out to become popular. Some of the more amusing things they did are here: http://www.cracked.com/article_19061_5-artistic-geniuses-who-only-became-great-after-selling-out.html
And lest we forget how much they cheated on their wives and more than enough other things that would make you puke blood. This is what disillusioning is like.