In order to keep visitors you have to give them a reason to come back, or rather give them a reason to want to come back. Also, People want to be able to find what they are looking for without any difficulties. If they look around and can't find whatever it is, they will leave and go to a different site in order to find what they want. Another thing that will keep visitors is offering your own newsletter. Read more about why to publish your own newsletter or e-zine .
People usually feel more comfortable with sites that offer newsletters.
There are many places that will 'power' your newsletter for free.