[TreyS]

Different types of diagrams highlight different aspects of a system.

If you can, try to put together 2 different diagrams presenting different views. For example, sequence diagrams, model diagrams, etc. each tell you something different. Having a few different perspectives on the same system will give you a richer understanding.

Not specifically related to visualization, but, also, if you spend a lot of time trying to understand a piece of code, once you understand it, document it (e.g. in a function or method comment). Over time, this makes a huge difference and will help you and others out when you revisit the code later.