REWARD: Something given in recompense for worthy behavior. For example: before you even start a practice session tell your child that he will get a prize after a good practice. He knows he can't act up or he won't get the prize. In the future, he will know that if he wants a prize he will have to have good behavior.
Start thinking about how you are addressing this issue with your children. I know I have caught myself wanting to say "if you..." but practicing (and other hard things for children, such as chores) goes so much more smoother if I offer rewards instead of bribes.