One of the first changes I made when I started focusing on group lessons, was to implement group challenges. I split my students into 5 teams, and began "Team Quest 100" - a challenge to the students to work together to learn 100 pieces of music by the end of the school year. All members of the winning team would win a prize.
I decorated the wall with a poster for each team so I could log progress after each lesson. When a student completed a piece of music, I wrote the student's name, the date, and the name of the piece on music on their team poster. It was easy to see how the teams were progressing.
For a piece to be "completed" and logged on the team poster, it had to be played to "Recital Level" at 2 separate piano lessons. So if a student had to restart, fix notes mistakes, or missed dynamics, the piece wasn't ready to be logged.
This was a big piano practice motivator. The first thing most students did on entering the studio, was to check the "Team Quest Wall" to see how their team was doing. The next thing they did was to check their log book to see how many pieces they could add to their team that day.
Another good thing about it, was that parents could also see the progress of the teams. One parent asked if the "Green Machine" team had less students because they were always so far behind. I explained that the teams were equal, and reflected the practice of the students in the team. She then proceeded to insist that her daughter practice more!
It was a very successful practice incentive. This year, I have a lot more students so I have split them into 6 teams and I am running a Team Quest 150 challenge where each piece has to be played to recital level at 3 lessons before it can be logged!