In a mixed planting, aim to include about 20 per cent ornamental grasses.
For the best effect, plant in a naturalistic fashion, threaded through other late perennials. Plants that associate well with grasses are often artless, wild-looking things: umbellifers, achillea, echinacea, eupatorium, persicaria, Verbena bonariensis and Veronicastrum, to mention a few. Fiery dahlias and red hot pokers contrast well with the muted tones of the grass flowers. If you have the space, plant a large area and weave paths through it, so you can float through the plants.
Most grasses get a bit flabby and are inclined to flop over in over-rich soil. However, very poor soil should be amended with garden compost or well-rotted manure. Water well at the time of planting. Dead foliage can be cut down in spring, and in evergreen varieties, raked out (although Calamagrostis arundinacea benefits from an annual haircut). If you want to increase late-flowering grasses, then dig them up and divide in late spring.