The best way to prevent this is by constantly updating your information. You may wish to open up new sections or just add more info on a certain topic. Newsletter and contest may be a lot of work, but they keep visitors coming back again and again. Newsletters are a good way of sharing information. If you can't publish your own, try to have articles in newsletters as often as possible.
Contest are a good way to attract visitors as well.
A promotional tool that every webmaster should use because everyone can do it is swap related links. Not only are you getting targeted links to your site, but it is a good way to satisfy your visitors also. Some people may come to your site just because you can lead them to where they can go. Interactive web sites let the visitors add content to the site as well as read it. Try having a question of the week, where on your site you publish a question someone sends you and answer it on your site.