Prepare the bed well by digging in lots of composted organic matter before you plant. Where possible, plant in odd numbers, and cluster them together to give impact. Multiples of 3 work really well set out in irregular triangles for an organic feel, while larger numbers can be planted in mercurial clusters or drifts.
In bigger gardens, try to repeat your clusters through the border to create a sense of rhythm and cohesion. Also, when setting your plants out, check their eventual spread and make sure they’ve enough space to fill out. After planting, keep plants well watered for their first year, particularly during warmer weather, and feed them with a good, general purpose fertiliser through the growing season.