Use structures to signal an entrance
Arches, gates, low fencing, or large containers flanking an entry all help mark a transition. I have used these elements throughout my garden. For example, after climbing the stairs from the street, the front gate marks the transition into a more private space. A small arbor or arch could be used to similar effect, requiring you to pass through it into the next space. In the backyard, I use a low, split-rail fence to identify the beginning of the perennial garden path as it leads from the large brick terrace. Its rustic construction signals that you are moving into a more informal area.
Throughout the garden, I've also used large containers—sometimes with plantings, and sometimes just filled with water for the birds—to highlight the point at which you move from one area to another. They also serve as focal points, drawing your eye through the garden, encouraging you to explore just a bit further down the garden path.
A simple change in paving materials can signal a transition. Stepping from my mulched woodland path onto the brick patio, you know at once that you are entering a new space. Whether your transitions are as subtle as the narrowing of a passage or as clear as a gate or arbor, emphasizing passageways is a creative way to highlight the changing moods and experiences in your garden.