Here is some info Will sent me a while ago:
Post-Holiday Care of Poinsettias: When the
flowers fade and the
plant no longer looks attractive, you can discard the
plant or you can test your green thumb by doing the following:
1. Cut the stems back to just above the point where new (green) growth is emerging. This usually means cutting off one-half to two-thirds of the stems. It may look ugly for a few weeks until the new growth has filled out.
2. Move the poinsettia to a sunny windowsill.
3. Do not repot the poinsettia.
4. Water and fertilize the poinsettia regularly. Do not let it get dried out. Avoid temperature extremes and cold drafts.
If you do all of the above you will be rewarded with a full, nicely shaped, green-leafed poinsettia. Keep up this regimen through the winter,
spring and summer. If you want your poinsettia to re-bloom, you will have to start preparing your poinsettia in the fall.
How to Get Your Poinsettia to Re-bloom: It’s not complicated, but it’s not easy either. Beginning in late September, you must provide your poinsettia with 12 hours of complete darkness each night and 12 hours of good light during the day. Even a small amount of light from a light bulb for a short time at night can disrupt the formation of the new
flowers. In addition, night temps should drop to 63 degrees. You must maintain this regimen for about eight weeks, by moving your poinsettia to a completely dark and cool place every night and back to the sunny windowsill each day. Once the colorful leaves called bracts are well established (usually about eight weeks), you can stop schlepping it to a dark place every night and leave it where it can be enjoyed. This is a lot of work. Most people prefer to purchase a new poinsettia each holiday season.
Good luck!