Deceiving the eye.
Curves, zig-zags or diagonal paths can trick the eye into thinking that the garden is longer or wider than it actual is. Horizontal lines make a garden look wider, whilst vertical lines make a garden look longer. If for example you have a long narrow garden, then plan a curved or zigzag path, a straight path down the middle will make your garden look longer and narrower.
If you have a small garden, don't include too many varieties of plants. Group a number of the same variety of plant together (this includes bulbs) and try to have the same colour scheme in a particular border. Single plants of mixed colours can confuse the eye. Note that strong bright coloured flowers foreshorten a garden, while softer colours lengthen a garden, create a false perspective by planting vibrant colours near to the house and muted tones at the end of the garden.
Make sure that your garden design flows from one area to the next. Try to balance the garden. Lead the eye from one side of the garden to the other with large staggered features (planters, fountains, trees, shrubs etc.), this will give the impression that the garden is larger than it actually is. Placing a large feature at the end of the garden is like a full stop, this is where the eye comes to rest, if you can incorporate a far feature, into your garden design, such as a view of a church spire, then your garden will again appear deceptively large.