To keep poinsettias for a long time start when buying it.
Plant should have green, no yellow leaves. Do not purchase plant in paper or plastic, will deteriorate quickly, or displaced close together.
After buying the plant WRAP properly, exposure to low temp. damage leaves. Place in sunny spot, away from cold glass and from drafts, open doors, windows. Keep temp 60-50 gr. Warmer = short life plant.
Watter every day, but DRAIN well.
Fertilize 1x p month afer flowering.
In febr, march cut back old flowering stems to 5 inches, repot in april, keep in sunny position. Afer frost put the plant outside in sun, shade after noon.
Turn pot every week: prevent rooting one side and getting even growth.
Like Eileen said: 3 months darkness is the trick.
At least from 5pm--8am between 30 sept and 15 dec.in the dark.
Hope you will be lucky this year.