Most home gardeners have a variety of trees, shrubs, and flowering perennials and annuals. Diversity ensures a lot of opportunities to have something always in bloom or for featuring eye-catching foliage or form. Yet, avoiding a mish-mash look overall is often a challenge for gardeners.
For example, I have spent many years buying one of nearly every new plant I have ever encountered. When one has lots of different plants, it doesn't always contribute to harmony and cohesion in a garden. I totally understand that having an overall design plan and adhering to it is often unrealistic, but I think one should make some attempt to tie the whole landscape together.
One idea is to consciously adhere to a preferred color palette. Usually this happens naturally as gardeners gravitate towards plants in colors that they like – but not always. So keep color in mind, both in terms of bloom and foliage. The same can be said for form and texture, and with these, occasional variation is helpful for accents and focal points.
Always use some repetition of colors, shapes, and forms to promote unity. Keep structural features, in terms of their materials and contours, similar. For example, if there are straight paths, make the edges of the beds straight also. And if brick edges some of the beds, try to repeat it in other beds and structures too.