Don't just down vote.
Avoid down voting posts out of spite or just because your opinion opposes what is being said. The point of a post is to have a discussion on the topic(s) mentioned in the post. When you down vote a post the post is made less visible to the community, making the post harder to see and less likely to be discussed about. Even if the topic at hand may be what you think to be terribly incorrect, you should first try to ask the author what was mean't or the idea he/she was trying to express, before you down vote. Give the author a chance to explain themselves before just hitting that down vote button.
This also applies for the authors as well. As an author of a thread, be mindful that people can express their views of your thread however they want. So as an author do not instinctively down vote comments who criticize or disagree with your opinion, and instead try and learn from it.
The only time you should down vote a thread or comment is when the subject is expressed in an immature,offensive, or irrational manner. Now, as I noted before even if the author expressed his idea in one of these ways still be sure to comment on why you down voted and how or what the author can improve on.
Thanks