Do you each Christmas buy a Poinsettia only to watch it die over the year, or, at most, have it live but never bloom again? These tips are from research over the web with some modification from my experience.
Water only when soil is completely dry (or close to it). A constantly moist soil will cause roots to rot and the plant will die. If you under-water, the leaves will begin to wilt, then water right away! Poinsettias are usually forgiving and will bounce back quickly.
Never let it stay out in temperatures below 65 deg F (I don't know the exact temperature, but Poinsettias will die if it gets too cold).
Give it as much sun as you can (except as noted below for Fall)
Give liquid fertilizer as recommended for house plants. Pinch off dead leaves, branches, etc.
Poinsettias are not very resistant to white flies. I later learned this the hard way when some flowers from a florist introduced them to my household. To protect the plant from this, I recommend using a mild soap water solution (or buy a mixture from your lawn and garden center that is basically the same thing with salts added) and spray it, coating the leaves (both sides), once a month. Don't do it too heavy, a light mist on both sides of the leaves are usually good enough. White flies like hanging around under the leaves, where they can easily suck the life out of the plant and stay out of casual view.
Spring (around May 5):
Pinch back all branches, leaving only 4 to 6 inches of the branches. This is also the best time to transplant. If you bought this plant this winter, you should transplant. The pots they sell it in are usually too small for the root system of the plant. I recommend using the same potting soil that is recommended for tomatoes.
Once you are *POSITIVE* that nights will not fall below 65 deg F, you can leave it outside in full sunlight. Do not place near parts of buildings where water will runoff and fall on the plant.
Bring inside once before nights get cold (below 65 deg F).
To make it bloom for Christmas:
The reason poinsettias don't bloom the next season is because the plant doesn't think it is Fall. The plant must have around 6 weeks of days with less than 12 hours of light before it will start blooming (thus think it is Fall). Typical indoor lighting is enough for the plant to be tricked to think it is still daylight.
Therefore, you must place the plant in a room with absolutely NO light for around 14 hrs per day. What I typically do is when I get home from work at around 5pm, place it into a dark closet. And, before I leave for work in the morning at around 7:30am, place it back into a sunny window. You should start this process sometime in between mid September and October 1. I recommend mid September so it will be blooming for Thanksgiving.
Once the plant has good coverage of blooms (not just some red leaves), you can stop placing it in the dark for the season.