Ok...this is how i've done it in the past. Bear in mond that the poinsettia is a small tropical shrub and ive seen it growing in barbados as an almost weed up to 6 feet tall.
Bearing this in mind, poinsettias need good light, warmth and most important, humidity. They will not appreciate direct sunlight through a south facing window....even in winter.
To keep them to next year, i grow them as a normal houseplant until late march. Keep them moist and let them lightly dry out between waterings....do not over water or allow to dry right out.
By the end of march, i would cut them back by half. Watering may be reduced as there will probably be no leaves on the plant...just stems. Also be careful of the milky white sap...it can be an irritant to some people. I would also give a dilute liquid feed at this point...half strength miracle grow is ideal.
When you see new shoots coming...pot the plant into a slightly bigger pot using general purpose potting compost. if the plan is in a 13 cm pot. you need to go up one size....to maybe a 15 or 17 cm pot. DO NOT OVER POT !!!
Then, keep it watered and fed throughout the summer and keep it misted to keep the humidity up and keep out of the sun.
around the beginning of October, you need to give the plant 12 hours daylight and 12 hours total darkness...and i mean TOTAL darkness.
is it worth it ? probably not but it's a bit of fun