An easy way to increase the number of plants in your garden is to divide its mature perennials. Plant division entails digging up a sizable plant and splitting it into two or more smaller individuals, each complete with roots, stems and leaves. These can then be sited and planted at any appropriate site in the garden. Division adds more plants to your garden for free, and in many cases these smaller, younger plants will be more vigorous and attractive than their parent. (For instance, division is often recommended when a perennial that flowered well in previous seasons ceases to do so.)
Division is most successful in spring and fall. Generally perennials should be divided in the season opposite from that in which they flower—so late summer and autumn bloomers are divided in spring, while spring and early summer bloomers are split in late summer or early fall. By dividing the plants out of their flowering season, you allow them to put all their energy into developing roots and leaves.
The key to success with perennial division is to make sure that the new plants are of substantial size. This holds true especially in fall, when they have less time to become established before the growing season ends. A plant that perhaps would have made five smallish pieces in spring may instead become three larger pieces in autumn.
The other vital detail for dividing perennials is aftercare of the new plants. When repositioning them, give them appropriate spacing so they have room to fill out to the variety's mature size, and plant them at the same depth as the mother plant was growing. If the days are still hot and sunny in autumn, you might protect the divisions for a week or so by setting up a sort of tent over them. This can be done with sticks or stakes supporting newspaper, a bed sheet or horticultural shade cloth or row covering. Be sure to keep the divisions watered well, just as you would any new plant in the garden, even if the plant is "drought tolerant." Deep, regular watering will encourage the plants to establish their roots in the soil.