*Set your environment up for success. Have the family ready to go and in a good mood (if possible.) Don't schedule photos or take them right before or right after a nap if you have toddlers. You'd be surprised how a little thought and planning will go a long way!
*Be realistic. Your children are not perfect, so their Christmas card photos may reflect that. It is okay. Roll with the punches--sometimes you end up with the best Christmas cards that way! Unrealistic expectations can ruin a fun photo shoot.And this one is not only good for creative techniques, but has a lot of other links as resources.