Truly the best time to plant garden perennials is in the fall. The reasons are many such as when you walk around your yard, you can see where you have "holes". You can easily envision what size and what colors you need to complete any particular area. In the spring, many plants haven't started growing so seeing what's missing can be difficult and you cannot depend upon your memories from last year to tell you what to get.
Late in the season most nurseries and garden centers have tremendous "clearance" sales and picking up a few new replacement plants could really be affordable. Also, plants at the end of the season are usually healthier with a better root system than the new, young plants that you purchase in the spring. The soil is also much warmer (and dryer) in the fall, even though the air may feel cold, and the growing season should continue until at least mid-November.
Keeping your new shrubs, trees and other perennials moist during the fall is much easier as the temperatures really cool down at night and the soil remains moist for longer periods of time. Fertilizing your plants in the fall months is usually discouraged, as fertilizer tends to produce soft, tender growth...
something you do not want just before winter. And remember not to do any early pruning or cutting back as this too causes new, tender growth.
Spraying your new and old plants such as azaleas, boxwoods, evergreens, hollies and laurels with an anti-transpirant such as Wilt-Pruf is a great way to lock-in the natural moisture that all plants have and to ensure that your plants survive the drying winds of winter. If necessary, you can install, on the west side of your plants, an inexpensive wall of snow-fence and burlap. This is a great way to cut down on the westerly winds of winter.