-forum notifications when your post is quoted, or thread has responses.
-newsletter? weekly, bi-weekly, monthly, irregularly, doesn't matter. People are signing up every day though, and I feel as though they forget to come back!
This is a good way to get members coming back. And well, how frequently they are sent out does matter because from my experience weekly & every day newsletters are annoying, and spam.
Just some ideas. Just throwing them out there.