All you need to remember are a few basic principles. Look for foliage types that complement each other. Generally a mix of small and large shapes that echo each other work, as well as a mix of bold foliage with fine or ferny leaves. I always like to start with the foliage because it's what you see most of the year, so I pay a lot of attention to the shapes, and even more important, to the colors . Foliage does come in many colors .
Greens tend to have either blue tones or yellow.
Or better still, use a plant with variegated foliage that combines the two colors. The colored foliage will add life to the composition even when nothing is flowering. Pay attention to color echoes. A collection of lots of different plants can be unified by repeating a few garden basics from time to time throughout the garden. For me, in this particular border, the basics are my bridge plant, the coreopsis, and the lamb's ears and veronica which form an edging that ties the whole composition together.