The most important criteria is to select the right plant for the right place. 
Make sure the cultural requirements of the plant match the conditions of the site. consider the following: 

Light
The amount of light your site receives is very important to the health and performance of you plants. Plants that require full sun, approximately 6 or more hours of sun after 10:00am, will not perform to their optimum with less light. They may bloom less, have smaller, less vivid blossoms, get leggy and floppy, will not be as vigorous, and be prone to disease. Plants that prefer partial shade will often experience foliage problems such as leaf burn in too much sun.

 Pay close attention to how much light your site receives, and choose plants that thrive in those conditions.

Soil Type and Moisture
The soil of your garden influences the health of your plants. While plants vary in their tolerance of soil types, moist perennials prefer well drained, humus rich soils. If your soil is poor, you can amend it with organic matter and fertilizer. Even with amendments, you may not be able to attain the perfect soil however. If this is the case, choose plants that are tolerant of these conditions.

 All plants need water to survive. Some, however, are more tolerant of dry conditions. If your site tends to be dry, you will need to irrigate or choose plants that are drought tolerant. Remember that even drought tolerant plants usually need supplemental irrigation until they are established. 

Hardiness
perennials are rated for their winter hardiness. Here in central Pennsylvania plants that are rated to zone 6 are considered perennial. Plants that are not hardy to your zone will need winter protection or they may not over-winter. 

Be Creative

 Once you have accounted for the conditions of the site, you can begin to be creative. This is where your garden begins to reflect your personal taste or style. Consider the following:

 Size: Height and Spread
Perennials come in a wide variety o heights and sizes. The eventual size of the plant you choose should be in proportion to the space allocated for it. Remember most plants will grow and need adequate space to perform to their optimum. So plan for their mature size. Most of the time you can find out how big a pant will get from available literature. These ratings may vary from source to source and the size attained will vary depending on cultural conditions.

 A good rule of thumb is to put the taller plants to the back of a border, behind shorter ones, thus making all plants visible. Remember height of plants varies with season, and many tall plants do not attain their final size until later in the season.

 With the wife range of perennials available these days, many plants have varieties available in an array of heights. There are several plants that have new dwarf selections available, such as the dwarf purple coneflower, Echinacea purpurea 'Kims Knee High,' the dwarf Bolonia, Boltonia asteroides nana, and many dwarf forms of ornamental grasses. 

Color: Flower and Foliage
Color is definitely a matter of personal taste. Whether you decide to stick with a single color, to use two or three colors, or go wild and embrace the whole spectrum of the rainbow is up to you. It is often helpful however, to decide what your color scheme will be so that you can tie your garden together.

 The color scheme you choose will set the mood of the garden. "Cool" colors, such as blues, yellows, and pins have a soothing effect. While "hot" colors such as reds and oranges are bold attention grabbers. 

Don't forget the colors of foliage. Leaves of perennials can provide a wonderful backdrop for other plants, but can also be a source of color in your garden. There are plants with solid colored leaves in shades of blue, green, yellow, silver, white, red, purple, and even black. There are variegated leaves with 2, 3, or 4 different colors all on the same leaf in a variety of patterns. Foliage can provide all the color you need. 

Shape and Form
Stimulating and intriguing gardens always depend on some diversity of shape and form. Perennials occur in countless different forms. They may be rounded, either loosely or in tight domes. They may be upright or prostrate and spreading. Interesting arrangements play these characteristics against one another.

 As important as contrasting plant form is to vary flower size and shape. Flowers can be borne singly, or in multiples. They can be spikes or spires, flattened or rounded, clusters, balls, daisies, stars, bell, cup or saucer-shaped. Foliage is equally diverse in size, shape and arrangement. The most interesting gardens take advantage of a wide selection of shapes and forms.

 Texture
Texture is created by the varying shapes and sizes of flowers and foliage, the interplay of light and shadow the background materials. fine texture generally feels more elegant and formal. Coarse is more casual or rustic. Complete uniformity of texture is almost always too dull. Generally you want to aim for some contrast - varying the proportion of coarse to fine to suit your taste.

 Bloom Time
A garden can be planned so that its beauty will span the whole year, with something happening at every season. Since most perennials have a peak bloom at a particular time of the year, choose plants with carrying bloom times for your gardens. Plan for a succession of blooms, with approximately one third of the garden in bloom in the spring, a third in the summer and a third in the fall.

 Through the efforts of plant breeders, there are now more perennials with extended bloom times or great re-blooming characteristics. You can extend the color of your garden by choosing some of these long blooming varieties.