In its scheme of things, nature has fashioned plants that are adapted to a day-length/night-length cycle. Taken away from it, they will not bloom or reproduce. Scientists call it photoperiodism.
There are three general categories: short-day, long-day and indeterminate. The poinsettia is a short-day plant. To bloom for Christmas, it must be kept in uninterrupted darkness for 14 hours a day from about October 1 until early December.
If you still have your plant from last Christmas (many do), keep it in a room where lights are not turned on from p.m. until 8 the next morning. Or put it in a dark closet every evening. There must be no light during the dark period -- just, turning the light on for a few secons will cause a delay in flowering.
The plant will not set buds and bloom unless darkness is continuous. The temperature during the day period should be from 60 degrees to 65 degrees F. If the temperature is too high or to low, the setting of buds may be delayed or halted.
(Other plants have similar problems. Ragweed, for example, sets buds when the day is 14 1/2 hours long and blooms in the middle of August. In Maine, the summer days do not shorten to 14 1/2 hours until August, and buds initiated then cannot produce mature seed before a killing frost.)
During the day the poinsettia should be given the best light you can provide. A sunny window is best, but don't let it touch cold window glass.The poorer the light, the less colorful the plant will be.
Protect the poinsettia from sudden temperature changes and keep it away from radiators and air registers when the heat is turned on.
Reduce the amount of fertilizer when the shortday period starts. With the lower light intersity, there is much less need for fertilizer.
The plant needs to be watered regularly indoors. The water should be at room temperature (75 degrees to 80 degrees F.) during the day. Keep the soil moist but not soggy. Usually this means watering every day or two. Water until it comes out at the bottom, wait about 20 minutes for it to drain, and empty the saucer.
If the plant wilts because you forgat to water it, immerse the pot to the rim in room-temperature water and leave it there until air bubbles stop rising fromthe soil. Then let the pot drain and empty the saucer.