Tony Buzan has defined these "rules" for Mind Mapping

1. Start in the centre with an image of the topic, using at least 3 colours.
2. Use images, symbols, codes and dimensions throughout your Mind Map.
3. Select key words and print using upper or lower case letters.
4. Each word/image must be alone and sitting on its own line.
5. The lines must be connected, starting from the central image. The central lines are thicker, organic and flowing, becoming thinner as they radiate out from the centre.
6. Make the lines the same length as the word/image.
7. Use colours – your own code – throughout the Mind Map.
8. Develop your own personal style of Mind Mapping.
9. Use emphasis and show associations in your Mind Map.
10. Keep the Mind Map clear by using radial hierarchy, numerical order or outlines to embrace your branches.

Attribution: Tony Buzan via Wikipedia

There is an article that is well worth reading about how the rules work out in practical use at the Mind Mapping Wiki. It discusses the rules one by one.