I believe that first and foremost, you need to understand the topic/question that you are trying to answer.  Try to answer to the best of your ability, and make it clear whether you are stating an opinion or basing your answer on facts, previous experience, etc.  If a question looks like it's asking for an expert-level answer, don't respond if you have no knowledge in that area.

Second, spelling and grammar are as equally important as understanding the question.  If you post a response that has a lot of typos or punctuation errors, it may cause readers to doubt how sincere you are in your response.  One or two little mistakes will probably be okay in most cases, but you should take the time to proofread every answer you write up before you post it.