When designing any plant bed, there are 3 main plants you'll want to focus on. Specimen plants (usually an ornamental tree), accent plants (roses or other shrub to provide color), and a key plant (larger tree/shrub to help house feel the right size).
I like an undulating flower bed that is less formal. I grew all the pink impatiens from seed in my basement under lights. My kids laughingly call it my grow op. Saves a lot of money and the plants are further ahead as I could not plant this year until late May. This picture is taken in early August and by late September the beds have been cleaned and are ready for winter and snow.
Keep Plants Vertical, Not Horizontal - Horizontal space is at a premium in many of the best small backyard ideas. That's why it's good to look for shrubs and trees that max out interest as they grow up, not out. Try dwarf varieties for a small backyard, as well as more columnar evergreens (bonus—they boost wintertime interest, too).