No flaming...be civil, even if you disagree strongly with what someone says. Don't post in anger, heated words only incite more flames, and then civil discourse--and thoughtful insight--are completely lost.
Proofread! Remember that this is an element of your AP English class, so proper grammar, spelling and punctuation are the bare minimum that is expected of you. Yes, you are graded on your contributions to the blog. Make it good.
Keep short comments to a necessary minimum. This is not your myspace page, it is not a place for you to chat with your friends. Your posts should offer a new thought or idea to everyone in the group, if you don't have anything to add, don't post. If you never have any new thoughts or ideas, you may be in the wrong class.
Sign your real name (or commonly known nickname) to all posts. Any anonymous postings or comments will be deleted.