Encourage the child to see him or herself as a unique individual with a valuable contribution to make to family and society. Never take away the thing or things that a child loves and succeeds in. This may be piano, sport, play or some other leisure activity. To say that a child can't do this until there has been some improvement at school is a sure recipe for disaster, and the resulting sense of deprivation and frustration will lead to even less achievement. Don't lecture or nag a child.
Reason is always preferable.
Don't pressure the child into doing something because you think it's a good idea. Don't set artificial times for work to be done at all costs and make the child feel that you are being a martyr. This reinforces the idea of failure, not only at school but at home as well. Be more natural in your interest and enthusiasm. Don't keep checking up on the child's progress.