A couple of times, I've had askers comment on my answer saying that it was right, but they don't take the extra step to mark the question as answered. I'd like to have something in the FAQ I can point them to.
The text might be:
When someone answers your question with a correct and/or useful answer, your next step should be to accept the answer that helped you the most, by clicking on the checkmark to the left of the answer.
This is important for several reasons:
It lets other users know that the question has been answered.
It moves the accepted answer to the top of the list of answers, so that when a user finds the question in a search, they can find the answer quickly.
Finally, it gives the user who answered the question additional reputation points. This rewards the user who answered the question, allowing them to gain additional privileges on the site.