Very few modern gardeners have herbaceous borders in the strictest sense. Most mix in a few other types of plants, such as roses and perhaps a few annuals. In gardening terms, there is very little difference between a mixed border and a herbaceous one. The shapes and design can be the same, and apart from using other plants, aesthetically the two borders are likely to look very similar in style.
Use a hodgepodge of plants in the border. There is very little point in being a purist and sticking to one type of plant when the whole object is to create an attractive border. It makes sense to use whatever is native and available.
Use shrubs to create a backbone for the border. A few shrubs will add a permanent structure to the border. Choose a strong herbaceous perennials like hydrangeas, then add perennials, such as petunias, with a shorter blooming season as accents.
Use the plant foliage to create interest and protect the low growers. The foliage of the shrubs can be used to set off the flowers of the plants in front of them. They can also provide a little shade for those plants that need it. Japanese maples with their stunning leaf colourations are a perfect choice for this method.
Mix perennials and annuals in raised beds. Treat raised perennial beds exactly the same as herbaceous and mixed beds, except be aware that they can be seen from all sides rather than from one or two. Consider using heathers for their strong structures mixed with lilacs for their beauty and fragrance.
Create a centre point in the bed. The main problem is that there is no background for the plants, so put taller plants in the middle, enabling the others to be seen against them. Shrubs or trees often make a good centre feature since they prevent the eye from running straight over the bed to what lies beyond. Japanese maples, with their lovely leaves, are perfect.
Accent the perennial beds with annuals. Using larger perennials in the centre of a raised bed allows the gardener to add annual accents to the sides of the perennials. This creates a dynamic garden by updating the raised bed for each season. For example, the lovely purple of Mexican heather always looks great planted alongside marigolds.