Have an open mind and don’t only think of negative commentary on a topic. Try to think of ways to improve upon the ideas that have been provided. Also, learn the background of what you are commenting on so you will not blindly be questioning the ideals of another person.
Read thoroughly and accurately so that you won’t have any misunderstandings. In addition, read other commentary thoroughly as well so that you wont be repeating/reiterating what another person said.
If you are in a disagreement, provide proper evidence and just for you reasoning, and make sure to understand your opposing side and why you oppose them and that any misconceptions that you have about them haven’t already been cleared up.