Recently I've been developing a large number of UML diagrams that are part of a Solution Architecture Document. I ran across a great little book that has helped me improve the diagrams style, consistency, and make them easier to understand.
The book is The Elements of UML 2.0 Style by Scott W. Ambler. The book assumes that you understand basic UML techniques and modeling already. It provides a set of standards and rules for different types of diagrams along with a justification.
The book is inexpensive and easy to use as a basic reference. Highly recommended.
