Spray the blades of your pruning tools with a household antiseptic cleaner. Wipe the blades dry with paper towels. This removes plant disease pathogens that you could spread to your poinsettia.
Put on gloves. Poinsettias release a latex-based sap when cut that can cause a skin rash.
Cut the entire poinsettia down to 12 to 18 inches from the ground with loppers or hand pruners. Loppers may be necessary when the lower stems of the poinsettia are woody. Cut any stems that were nipped by frost all the way back to live wood -- scratching into the bark with your fingernail until you come to wood with green behind the bark. Do this in spring.
Prune the stems back again, leaving four leaves on each shoot when the plant has put on 12 inches of new growth. Two new shoots emerge from each cut.
Pinch back the tips of terminal shoots with gloved fingers each time a new shoot puts on 12 inches of growth. Stop in mid-August or you risk removing the buds that form the colorful bracts and tiny flowers for the plant's winter show.