10 more rules:

1. Never go out without a notebook
2. Find one mentor - or many.
3. Do your homework.
4. Trust Your Own Experience.
5. Don't get hung up on plants - A Garden is bigger than all that.
6. Never think you'll get it right the first time.
7. Encourage Self-Seeders.
8. Remember, Sunlight and Shade are Design Elements.
9. Avoid Fussiness.
10. Focus on the Garden YOU want.
(Garden Design's Dorothy Kalins' observations of Penelope Hobhouse)
Related:
Penelope Hobhouse's 10 Rules of Landscaping:
My 10+ rules (of landscaping)