One way to design a flower bed is to focus on combining different plant forms. Some plants have fountain shapes, some are very upright or spiky, some are roundy-moundy and others are low and spreading. A nice design often makes use of several of these forms together.
Here's an example that combines 4 different plant forms for a beautiful late spring show:
1. tall upright - violet siberian iris
2. tall rounded - pink peony
3. fountain - white 'Joan Senior' daylily
4. low mound - reddish-coral heuchera (aka coral bells)
In the above example, you'd want to put the taller siberian iris and peony in the back or middle of the flower bed, with the daylily and coral bells toward the front or edge. Depending on the shape and size of the flower bed, you might plant groups of 3 of some or all of the plants. You could create a beautiful border by repeating this grouping of 4 plants several times in a rectangular bed.
Here is another group of plants using this design, but these would be good for a sunny summer border.
1. tall upright - green/magenta/brown 'Green Envy' echinacea (coneflower)
2. tall rounded - red/crimson 'Darcy Bussell' english rose
3. fountain - yellow 'Stella d'Oro' daylily
4. low spreading - lavender 'Blue Clips' campanula (bell flower)
And finally, this grouping would be perfect for a shady area:
1. tall upright - white 'Honorine Jobert' japanese anemone
2. tall rounded - pink or blue 'Endless Summer' hydrangea
3. medium mound - green/white-leaved 'Patriot' hosta
4. low mound - burgandy-leaved 'Palace Purple' heuchera