For my own use I formulated this rule of thumb: What is straight, should be curved, what is curved, should be straight. Meaning: in a garden where everything is straight, the walls or hedges around it and the path through it, the secondary landscaping should be curved: sloping or freakish paths, hedges, lawns or borders and the other way around: in a freakish or shapeless garden the secondary landscaping should be straight, in order to obtain a harmonious image.
The idealised harmony of a symmetrical garden with straight hedges, an elongated straight pond in the middle and trees planted symmetrically on both sides (that never grow at the same speed: farewell symmetry), has outlived itself. It is a theme from the past, when people used to be afraid of nature, which was ubiquitous, and the fence of the garden was meant to lock nature out. Today, when nature around us has virtually disappeared, the chilly, nature-adverse surrounding has to be kept outside the fence. Logically the internal garden has to be landscaped in a different manner than it used to be, because nature is not symmetrical. Freakish, curved or round, reflecting the idealised harmony of nature. Idealised, because harmony only exists in our imagination, not in nature. The garden remains a human creation: art (or kitsch): an illusion.
The planting has to be founded on a lucid concept, which can be understood at first glance.
Choosing a theme is the easiest way, though hardly natural: vegetable garden, herb garden, a single coloured garden . The concept of the perennial border, as developed by Gertrude Jekyll, was much more sophisticated. A natural theme, the flowering roadside or woodland edge, could in that way be drawn into the limited space of the garden.