Make posts that aren't spam and contribute to the discussion at hand, that will avoid negative karma for the most part. To get karma though you're going to have to make posts that set the bar a little bit higher, usually at least a paragraph worth of text as well. Make a solid point or argument or theory, bring something new to the discussion, etc. Just try to make the forum to be a place you actually want to spend time, not just a job or task you're set to accomplish.