Before you can market effectively to your existing customers you need to have some kind of CRM (customer relationship management) database to track their details and buying profile.  You don’t need anything fancy or expensive – an excel spreadsheet is better than nothing at all.  You just need something that enables you to track anything pertinent to your sales/marketing efforts.  Who is the purchasing contact?  What other services might they have a need for?  When was the last time you called them to check everything is OK?  This is the kind of data that you can actually use to your benefit.