The best way to determine what you can do to make visitors come back time and time again is to ask yourself,
"Why did they arrive at my site to begin with?"

Was it for content? If so, keep adding more!
Was it for tools or feature? If so, keep adding more!

See what I'm saying? Find out what's working for you and then expand on it. Also, like I said earlier, visitor interaction with your site is helpful in letting them know about your updates. RSS Feeds, Twitter, MySpace, Facebook and a Mailing List... almost endless possibilities.