Proper care starts when you bring the plant home from the store. Poinsettias are sensitive to cold temperatures. Even temperatures in the low 40s can cause leaves or petals to drop. If it is a long distance from store to the car, cover the plant with a bag or sweater.
More poinsettia plants are damaged by overwatering than underwatering. Before watering, check the soil with your finger to see if it is still moist on top. Apply enough water so a little runs out the bottom of the pot. Do not let water accumulate inside the foil wrap. You may remove the wrap and let the water drain into a saucer or the sink, or turn the plant upside down to drain water a few minutes after watering.