About me, I go with an explaining comment which usually goes like:
Hello, welcome to Superuser(SO/SF).
Please check the FAQ
(http://superuser.com) about the way
this site works. This site is not a
discussion board, this is a place for questions
to be asked and answered.
Then you add the appropriate explanation, depending on situation, like for example
If you want to ask a new question, do
it by clicking "Ask Question", to be in
a separate question.
Or
If you want to answer to someone else,
do it as a comment under his question.
Commenting requires 50 reputation,
review the FAQ about the reputation
system.
Etc.
And bottom line:
If what you want to write doesn't
answer the question, don't post it as
an answer.
Keep in mind that the way this site works is not obvious to new people, so I think an educational approach is the preferred way to handle things..