I follow a simple rule to get my child to concentrate. Give attention to get attention. How much attention a child gives a task also depends on whether he is enjoying himself. Many children struggle when asked to do something they dont't want to do. What I suggest you do is to turn things that is more important to you than to your child, into something fun for her. If you are trying to teach her how to write well, train her to draw on a chalk board or paint out letters rather than forcing her to sit down and repeatedly write long sentences.
Hope this helps!