Avoid repeating the same exercise more than a few times during home training sessions and vary the order of exercises. Try to end each exercise when the dog is doing well, even if it is only a small success. Then go on to the next exercise. Keep the sessions light and lively. Home training sessions should be kept short, about 10 minutes, and held 3-4 times a week.
Many trainers make the mistake of overworking their dogs.
I've found an occasional week off from class and an occasional week with only one or two practice sessions increases exuberance and interest. Assume the dog does not understand what you want, not that the dog is rebellious or stupid. Stop, get feedback from your obedience instructor and analyze what is happening. Go back and start teaching the exercise from the beginning. Try to avoid training when you're feeling stressed or tired.