ignatz,
There are several ways you could go about this, using an app like SmartList to Go by
www.dataviz.com or HandDbase by
www.ddhsoftware.com to built a custom database and forms.
You could also use Agendus by
www.iambic.com where you could add each client as a contact and make journal entries allowing you to track what, where and when you did something for each client. Another plus about Agendus is that it integrates with Mapopolis. You can store a location or address entry for a contact and click an icon that will open Mapopolis and take you to that address or set it as your destination for turn-by-turn navigation, with or without a GPS.