Here are some tips for successfully planting/growing your garden treasures:
Always harden off plants or purchase ones that have been hardened off. (See gardening wisdom on hardening off plants.)
Always water your plants before planting. A dry root ball is difficult to get wet after planting. If the plant is very dry, soak it in a bucket of water.
Prepare the soil, digging a generous hole and adding compost or peat moss to improve the soil.
Teasing the roots apart is not necessary unless the plant is very root bound. If the plant is very root bound, I like to run a knife down the side of the root ball in a couple of places, cutting about 1/8" deep. Typically, a plant that will aggressively fill a pot with roots will also aggressively "take off" once planted; so don't get too worried about the plant being pot bound.
Plant you plant so the root ball is just at the soil level or a little below, so the potting soil is covered.
Sprinkle Osmocote timed-release fertilizer around your plant, being careful not to get it into the crown of the plant.
Water your plant in very well; it may take more water than you think to get it good and wet. Placing the plant in a little depression to hold water may help.
This is the important part! Water your plants well every day for the first two weeks to get the roots to root out into the surrounding soil. This is where many people fail. If the plant is not watered enough every day for the first two weeks, the root ball may dry out and the plant may not grow well.
This is another important part! After the first two weeks, water your plant well (deeply) once a week any week you don't get an inch of rain. This is essential for perennial establishment.
Container plantings take more and more frequent watering than bed grown plants.
Keep your plants groomed, removing spent blooms and "finished" foliage. Trim or pinch as needed to get a bushy plant.