Planting perennial flower beds is the best way to ensure that your garden will always have some type of bloom, and because perennials can last for many years, a properly planned perennial flower bed is a good way to reduce the amount of effort you have to put in over the lifetime of your garden. Perennials don't have to be replanted every year, so planning a perennial flower bed differs somewhat from dealing with a bed for annuals or bulbs.
Dig up the sod and any other plants and weeds down to about 6 inches in an area approximately three times as long as it is wide. Because perennials don't all flower at the same time, like annuals, it's important that a perennial bed be large enough to always have a good distribution of flowers from the plants that are blooming at any given time.
Till or turn the soil thoroughly in the area you have just excavated.
Replace the soil and sod you've excavated with a clean topsoil so that the soil is up to 1 to 2 inches of the surrounding ground.
Observe the light, moisture and soil conditions in your new bed before selecting and planting perennials. If your flower bed gets a lot of light and little moisture, then you'll plant entirely different perennials than you would in a shady, moist bed.
Plant at least a half-dozen kinds of perennials, and think about when they will flower so that you always have something in bloom. Plant densely. The less soil you have showing, the better. Also use a short ground-covering plant at the edge of the bed to provide a smoother transition from the flower bed to the edge of your lawn.
Consult with your local nursery for groupings of perennials that work well in your local climate.