First things first, Poinsettias hate draughts. If they catch a chill, they’ll drop their leaves in an instant. So, when buying your plant ensure it’s a healthy looking one kept in warm conditions. This may sound extreme, but when taking the plant out from a warm shop to your car put a plastic bag over it to minimise the temperature difference shock.
Once home, keep it in a consistently warm, light room, out of bright light and the dreaded draughts. Don’t overwater, instead wait until the compost is dry before watering.