Keep soil evenly moist at all times. Even a day without adequate moisture is enough to make poinsettias drop their leaves. Provide a constant source of humidity by filling the overflow saucer with gravel. Water seeping through the pot will evaporate from the gravel. Turn up the heat.
Poinsettias are native to Mexico, where it is hot during the winter.
Keep the temperature as close to constant as you can, day and night. Decreasing temperatures cause leaves to drop. If leaves begin to drop, increase humidity and reduce water. Poinsettias make wonderful compost at the end of the holiday season. Inc. How things get done.