September's issue of Sunset magazine had some great examples of how using geometry can open up a small garden. We saw it in action in the neighbor's front yard, jump below for some more examples:
Using square pavers and rectangular garden paths is one part of creating space. The other? Texture. When you use small texture (like little pebbles in a path or underneath the table and small leafed plants with variegated leaves) it tricks the eye into making it seem further away. Creating different heights in a small space helps too. So planting a vine up a wall or a raised planting bed keeps the eye moving around and you feel immersed in a space rather than just looking at it.