I read a great idea for encouraging young children during piano lessons or practicing. I tried it with Curious George and it is amazing! And so simple! Here's what you do:
Put 10 m&m candies (or marshmallows, skittles or whatever small candies your child loves) on the piano during the lesson or practice session. Explain that your child or student can have all 10 treats as long as there is NO complaining during the lesson (or practice session). Every time they complain or whine (or whatever negative action you are trying to discourage), you simply go over to the piano and eat one of the candies. You don't say anything, you just take one. Pretty soon they catch on and stop complaining. It's great!
In fact, I've even taken it a step further. Now anytime during the day when Curious George is getting whiny about something - I just reach out into the air and grab an invisible m&m and stick it in my mouth. He thinks it's hilarious and we both end up laughing about it. But he really has been more conscious about not whining as much.