Perspective is often explained by referring to the fact that parallel train tracks appear to draw closer as the distance from the viewer increases, eventually appearing to meet.  I’m not suggesting that you should lay train track in your garden, (unless of course you’re a model rail enthusiast!) However you can use this technique to create ares of perspective.  You can for instance use line and and defined elements of volume to achieve this.
Perspective and Focalisation at Altmont Gardens.

Photograph Shirley Lazenby

Focalisation guides the spectator towards something in your garden design that you want to draw attention to. This can be achieved by identifying the vanishing point, or other elements of your perspective, and then placing your object there.  

The vanishing point is the theoretical point at which lines in a perspective drawing will disappear.  By placing your object or activity area at or close to this vanishing point you will draw attention to that object or activity area.   

Straight line will allow the viewer to see things more quickly than curved lines & both can be used for different purposes.  For instance if you have a large front garden and you want people to find your front door quickly and easily, then use your materials in reasonably straight lines.  If you want to discourage people paying immediate attention to other elements in your large front garden, then use curved lines.  In your rear garden perhaps you want people to arrive quickly and easily at the pool area but you want them to take a longer route to the vegetable garden.

Focal Point - a person at the end of a rose walk.

Transition of plants or other objects along these lines can strengthen or weaken the focalisation. Since focalisation is employed to draw attention to a particular point, people are naturally drawn to that place. Therefore, focalisation could be used to direct traffic in a space, or guide the view toward features to which you attach a particular value.

The photograph above shows the rose walk at Le Potager Extraordinaire, Le Mothe Achard, France. This uses simple focalisation to draw our attention to a person at the the other end of the walk. The focal point and focalisation are related; the focal point simply refers to one entity while focalisation combines a number of elements that are at your disposal.