Try not to expose your plant to cold or hot air drafts. Be sure to supply your poinsettia with adequate, but not excess water. Apply water when the soil feels dry, changes from a dark to a light color, or when the pot feels light if lifted. Soak the soil completely to the bottom of the pot, and be sure that excess water can drain freely. If the plant pot is covered with foil, punch holes in the foil underneath the pot so that water can drain out of the pot properly.
Poinsettias are notorious for dropping their lower leaves in the home.
Norfolk pines and spruces are often displayed at Christmas with lovely decorations. North windows are not bright enough. Even moisture is important. Don't keep soil too wet, or root rot may set in. Underwatering may show up as browning on leaf tips.