Angela G has some great suggestions.
The trick is really getting them to see that it matters - it's not punishment, it has a purpose. Everything they do, whether they see that it has a long-term benefit or not, is working their brains and showing that they can and will work as a part of a group (the class). Doing something that's difficult has a benefit. Doing something that seems easy has a benefit because practice helps to cement that knowledge. Everything works the brain cells. Find out what their goals are (even if they are possibly childish or unrealistic) and show how brain power is important in that area. But it can't just be about being good in one subject. For example, if one wants to be an astronaut, he may feel that he only has to do science and not reading. But NASA wants people whose entire brain works, not just those who have some science facts. They need the parts of the brain that are stimulated and exercised by reading, they need people who speak a foreign language (to deal with international space crews), and who can read complicated instructions as well as figure out mathematical complexities. If a child wants to be a professional athlete (as unlikely as this is as a career goal), you say, "Fine. Athletes need to be physically fit, but they also need math to figure out percentages and stats, they need incredible reading ability to be able to read contracts with managers and teams and to protect their millions of dollars in salary, they need to do exercises and thousands of hours of training time doing stuff they think they don't need because they're good at it." Every career is like this.
The next step is for them to accept that this is their JOB. Their job is to get an education, and within that, to have the best possible time doing it. They can make it take longer by being stubborn or inattentive, or they can maximize their time by being focused and organized and efficient. There's an expression that goes "There's never time to do it right, but there's always time to do it over." So people who don't apply themselves at first wind up spending twice or 3 times as much time doing something over again.
The kids will have more time to do fun things if they don't whine or back talk you, if they don't go off on tangents instead of applying themselves, and if they keep their papers, assignments and their desk in an organized fashion.
If they don't do their homework without you nagging, let it go. Let them have the consequences in school. It's okay if they go in unprepared at this age (or at any age). If they have to stay in from recess or spend the weekend doing make-up work, fine. Sometimes kids have to learn that way, and to realize that it is what they CHOSE. Repercussions and consequences are often better teachers that parents who nag or remind or ride herd on their kids. (I'm not saying you do these things - just talking in general.) The kids have to accept some responsibility. So if you can really help figure out each one's learning style and strong/weak points, you will do far more for them than you can imagine.
When they begin to have a sense of accomplishment, it's a great reinforcer.
