Use package diagrams for higher-level diagrams.
Prefer single-page diagrams.
Keep diagrams simple. Use 7+-2 elements rule.
Use horizontal/vertical lines, not round or diagonal.
Size of the element on the diagram represents its importance (generally).
Packages should have short descriptive names. Break elements into packages up to the point where you can assign short descriptive names to the packages.
Use colors to separate elements on a diagram.
Allen Holub's UML Quick Reference (link)