General Diagramming Guidelines

• Bubbles represent diagram elements such as class boxes, object boxes, use cases, and actors.
• Lines represent diagram elements such as associations, dependencies, and transitions between states.
• Labels represent diagram elements such as class names, association roles, and constraints.

Figure 1. Depicting crossing lines.

Figure 2. Improving the attractiveness of a diagram.

Figure 3. Indicating uncertainty on a diagram.

1. Avoid Crossing Lines
2. Crossing Lines Jump One Another. When you need to have two lines cross one of them should “hop” over the other, using the notation that you see in Figure 1 borrowed from electrical-wiring diagrams.
3. Avoid Diagonal or Curved Lines
4. Apply Consistently-Sized Bubbles. In the first version of the diagram in Figure 2 the A bubble is larger than the others, drawing attention to it.
5. Show Only What You Have To
6. Prefer Well-Known Notation Over Esoteric Notation
7. Reorganize Large Diagrams Into Several Smaller Ones
8. Include Whitespace In Diagrams
9. Focus on Content First, Appearance Second
10. Cleanup to Rethink a Diagram
11. Organize Diagrams Left to Right, Top to Bottom
12. Set and Follow Effective Naming Conventions
13. Apply Common Domain Terminology in Names
14. Only Bring Language Naming Conventions into Design Diagrams
15. Indicate Unknowns with a Question Mark. Figure 3 depicts two examples.
16. Consider Adding Color to Your Diagrams

Source

Material for this article was summarized from Chapter 2 of The Elements of UML 2.0 Style.