If you are one of "those" gardeners who could not bear to throw away your poinsettia or Christmas cactus last year; and you are entertaining the notion of these plants flowering in time for the holiday season, you need to take certain steps now. My assumption is that you took proper care of the plant during the spring and summer and it is looking pretty fine right now.
To force the poinsettia into bloom*, give it 14 hours of complete, uninterrupted darkness every night for six weeks, beginning in early October. Your poinsettia must be kept completely, completely dark from 5 p.m. to 8 a.m. Put a bag or a box over it and put it in a closet that you will not open. Start this treatment right now and plan on continuing until about December 15. Once the color starts to show, continue darkness until the bracts are almost fully opened. Temperatures should be no less than 55°F at night, but not more than 70°F. During the day give the poinsettia as much sunlight as possible. Of course, you will continue to water regularly and very lightly fertilize the plant. Don’t forget to bring the plant back into the light every morning.
Do the exact same thing with Christmas cactus except they need to be cool at night; 50 degrees is ideal; perhaps a dark garage and again no light at all.
Good luck, dear gardener, I hope you have blooms galore. As for me, I am not one of "those" gardeners. I love the feeling of purging that comes from throwing the dusty old plants away. I am always ready for the holidays to end and having these plants sitting around depresses me. I will purchase a new lush plant at the garden center in December, enjoy it through the holidays and take great pleasure in heaving it into the compost heap on January 1!
*The red, yellow, or pink on the poinsettia plant is not really a bloom as such. It is a bract or leaf that changes color with the introduction of darkness. The flower is the little bitty yellow in the center of the bract.
Here's all you'll need to know:
Poinsettias Fact Sheet
http://www.ext.colostate.edu/pubs/Garden/07412.html