Penelope Hobhouse's 10 Rules of Landscaping:

1. Rely on Geometry
2. Enclose an Oasis, create a paradise.
3. Build rooms with plants - trees, shrubs...
4. Foliage before flowers.
5. Reach for the Sky - trellises, arbors and climbers
6. Plants not politics. No PC.
7. Frame the View - borrow scenery from the neighbors.
8. Repeat Color and Texture.
9. Repeat Simple plant shapes - rows of trees, etc.
10. Unify with ground covers
(Garden Design - she (Penelope Hobhouse) comments that "In America - the natural setting for each garden assumes greater importance than it does in Europe," and "Every Garden should have some element of mystery in its design."

Related:
10 more rules:
My 10+ rules (of landscaping)