If you love flowers but have only a thin sliver of a bed to plant, the good news is that constraints such as this can actually make landscaping easier. In small spaces, the edges of the bed act as a frame similar to the edges of a painting or photo, making the space inside feel more unified. Investigate several possibilities for filling the frame.
Simplicity Is Best
Stick to one style of planting, whether it's cottage garden lush, Japanese spare or tropical bright. Repeat plants along the bed to give it a feeling of unity, or use a single specimen, perhaps a vine, with complementary plants around it. You can, of course, repeat combinations of plants, perhaps a grouping including a hosta, an astilbe and a Siberian iris, separated by groundcovers. Don't restrict yourself to your "theme plants," especially if you're creating a cottage garden mood, but use enough of them to make the bed feel similar from one end to another.
Vines are ideal for narrow beds, giving a sense of height and privacy. Clematis are excellent vines for small areas, as are climbing hydrangeas. Larger vines such as Campsis and honeysuckles can be used, but be sure to prune them back to a few major branches every year to keep them from taking over. Don't discount trees, either. You can take a flowering plum, for instance, and prune off the lower branches as it grows, encouraging it to spread out above head height. Or you can tie and train the branches to lie flat along a wall or fence, a process known as espaliering.
Textures and Contrasts
Pay attention to the textures of your leaves and flowers. Hostas and dahlias, for instance, are large and bold. Astilbes and yarrows, on the other hand. are small textured and ferny. Placing plants with contrasting textures together gives an immediate sense of interest similar to placing contrasting colors together.
You can also place rounded rocks with squared flagstones, a smooth, glassy ball next to a rough piece of driftwood, and so on. You have a limited area to play with, so make every piece of the design count.
Plants grow, change, flower and seed, die back and shoot out in odd directions. Stones, sculptures, driftwood and other non-living elements of your design will stay where you put them and, in general, keep their colors and sizes. As a rule of thumb, make sure that your composition looks great before you plant, simply by using interesting hardscape, a term often applied to these parts of the design.
This includes, of course, the paving of paths and walkways. If possible, make these pieces of art as well, using natural rock, gravel, river stones, interestingly shaped pavers and other materials. The paths unify your bed so make them interesting.