Ask HN: Tips on sketching diagrams/mind-maps?

I often sketch diagrams when trying to understand something, such as a software system or mathematical proof, but my diagrams always end up messy and inconsistent. Formal diagram systems (e.g. UML or ERD) are generally too complex or domain-specific for quickly mapping things out on paper.

Do you have a general approach to sketching out a concept or system? Even if not, miscellaneous tips and rules-of-thumb are welcome.