I realise that I am only new here but perhaps I can give a few pointers.
First and foremost, think about what you write. The way I usually go about this is read posts in the morning, write my replies but hold off posting unless there is something that needs to be addressed and come back to it in the evening. That way I can think about what I had written and look at it again to make certain that I am happy with what I have written. Now not everyone has to do it this way but it lessens the chance of something coming up and biting you later on.
The second point would be that you are not writing an exam. People tend to think that the longer something is the more it tries and justify itself, and if it is on someone being wrong then it looks like one big whine. By adding to a post and adding to a post and adding to a post not only could there be more in what you write for others to attack with it bores people who think you are bragging about how smart you think you are.
You should never ever ever ever take what you read too seriously. To use an allegory if you were a black girl and someone was criticising black girls, then the message that comes across is that it is not cool to be a black girl. Really that is their problem and all they do is come across as a jerk broadcasting it.
And most importantly, at least try and remember that you are dealing with people who do not necessarily share the same views you do. This actually ties in with point three in not upsetting others. Whether it be racism, politics, or religion, quite simply there is no need for attacks on others because of what they think.