Newsletters are a great way of keeping in contact with your clients.  Allow visitors to subscribe to your newsletter somewhere on your home page.  In the case of .html site link this to a reply form.  In the case of CMS sites, you can install a widget where visitors can subscribe to your feeds.  Then whenever you publish an article they will get an email informing them of this.  Use the newsletter to keep clients up to date with new products/services, specials etc.  It is also a good way of educating clients to the point where they realise that they need your product/service.