Your website is your shop-front.  While its primary function may be to attract sales enquiries, it is also a great tool for communicating with existing customers.   Add a customer page to your site, containing the information your clients need to get the best value from your service.  Provide guidance and advice relating to your industry (a file storage company, for example, might offer document retention guidelines).  Establish your company as the expert in your field, and alongside this, don’t forget to cross/up-sell – publicise regular sales promotions aimed specifically at maximising spend from existing customers.