Traditionally plants are placed in the border with small plants at the front, medium ones in the middle with tall ones at the back.
This works well because the smaller plants are not casting shade on the plants behind.
But it can end up looking a bit static. Like primary school children lined up for a class photo.
Its possible to introduce a more naturalistic planting by adding high plants that don't have bulk therefore don't cast shade.
Spire shapes such as this Digitalis (Foxglove) are perfect to use at the front of a border. Despite their height, they are elegantly slim, self supporting and their foliage bulk stays low. Foxgloves or other Spires will create a sense of rhythm when repeated every metre or so. Don't use so many you end up creating a row. You want vertical accents only.
Another useful plant shape are Dots. Slim, cloud like plants, again where the bulky foliage stays low. Flowers are dotted about on slim, high stems giving a transparent effect. Verbena Bonariensis is a very useful Dot plant. These can be used front, middle or back. Verbena's are a statement plant so I tend not to repeat them in a row. Scatter them through the border using different border positions.