Considering mapping out your classes using Class-Responsibility-Collaboration cards (wikipedia link.) It's a technique used to help you determine what classes you need and how they will interact with each other. Even if you have an existing class structure, the questions you'll answer by going through the CRC process is valuable information that should be explained in your documentation. The way classes interact with each other is one piece of information that is not usually explained well by documentation but is immensely useful in understanding how or why or when to use a class.

If you have a complicated class structure and you want to simplify or generalize it, tell us more about it, and perhaps we can offer some suggestions.