Plant in drifts rather than blocks. In very formal gardens, repetition often is used. Plant collectors, however, may repeat color combinations but, since they are by nature acquisitive, will use a wide variety of plants. Think of green as a color since all the flowers will not bloom all season. Use grays, dark and pale greens, and, with restraint, variegated foliage.
Color is, of course, most important.
Having designed scores of borders, I have found women will ask for pastels, men for strong colors. Gertrude Jekyll, probably the most important garden designer of the 19th and 20th centuries, used a color progression as well as sections thereof. Her progression went from the palest pinks and yellows, growing stronger until scarlet and gold predominate, then bringing in blues gradually with blue-reds and purples and bright blues, then softening again through lavender and pale blue and white. For best effect, the border should be at least 8 feet wide. If it is backed by a wall or hedge so the back can't be reached without stepping into the garden, stepping stones can be laid throughout to allow working without compacting the soil.