|Always have a back-up activity ready when you start a practice.
Although teachers may disagree, I believe that sometimes it is just too difficult or boring to play the assigned lesson. But I am sure that we will all agree that it is better to play something than to skip a practice altogether. And practicing when you are not in the proper mood is not effective.
Rather than fight through the lesson, move to another activity for a few minutes just to change the mood, then return to the teaching point of the lesson in the form of a game. After two or three attempts to put the lesson back on track it may actually be best just to end the lesson a little early.