The biggest mistake is setting plants too closely together. By the second season they are hopelessly crowded. Instead, plant in a 5-year concept. In the early years, use bold annuals to add drama and bulk. Just a handful of sunflowers and nasturtiums gives you an instant sort of effect.
Repetition is a beautiful thing.
If it's worth planting once, it's worth repeating. Cool it with the colors. Americans use too much color in their gardens, which often results in a garish overstatement. Think fewer flowers and more foliage, especially with unusual textures and shapes. Don't be a wanna-be.