Ball Mark Repair 101

Use a pronged ball mark repair tool (preferably), knife, key or tee.
Insert at the edges of the mark-not the middle of the depression.
Bring the edges together with a gentle twisting motion, but don't lift the center. Try not to tear the grass.
Smooth the surface with a club or foot. You're done when it's a surface that you would putt over.
Source: Golf Course Superintendents Association of America