I used to use a rather bizarre bit of positive reinforcement. I am a pretty solid cook, and was in the habit of occasionally bringing in food.
I make a mean rack of ribs, and I would occasionally say "First rehearsal that no one calls for line, I will bring in ribs next rehearsal."
Cast members quickly self-policed!
I also used to let parents know, in no uncertain terms, that lines were homework. I likened it to free throws in basketball. That is a skill to be practiced on one's own, not one that consumes team time.