Questions
Vote up a question if you think it's a valid question that will make someone's life easier. A good rule of thumb is: if you are answering and/or are interested in the answers to a question, vote it up. If a question duplicates a previous question, vote it down and leave a question comment letting them know it's a duplicate (include the URL of the original).
Vote down a question if it is a poor fit for Unshelved Answers. This isn't a discussion board, so we shouldn't have questions like "How was your day?"
Answers
Vote up an answer if you think it is correct and/or useful. A good rule of thumb is, if you're nodding at an answer, or learned something from it, vote it up. If an answer duplicates a previous answer, vote it down and leave a comment letting them know it's a duplicate.
Vote down an answer if it is factually incorrect, not just because or if you happen to vehemently disagree with it.
Comments
Vote up a comment if it helps clarify things.
Vote down a comment if it makes things murkier.