court, i'm there with you! here's my take on it for what it's worth.
for starters, i think THIS age is the perfect age for them to learn the consequences of doing poorly, because, let's face it, their grades in fifth/sixth grade really don't matter in the grand scheme of things. BUT it's a great place for us as parents to teach them about the way life really works (from which, up until now they may have been a wee bit sheltered).
sebastian has done this a number of times so far... come home without his assignment folder or necessary books or whatever. or else waited until the day before to start working on a big report. grrr! as an inconvenienced mom i want to throw a fit quite frankly because it sucks to be scraping things together last-minute. and i KNOW he is a super smart kid who could be getting great grades rather than the meh grades he gets when he's lazy or forgetful.
we've kind of tried to use it as a chance to help him understand life a little more than he does. like, asking him to think/talk through what he thinks would happen to daddy if he showed up to work without his tools to do his job? or what if he had a lazy attitude at work? what would be the consequence of that? what would happen long-term if he were to lose his job (no money, may be harder to get another job, etc). i mean, we don't try to scare the kid, but for him it really helps when he can understand WHY we are asking certain behaviors of him rather than just blindly accepting the expected behavior and complying. we have basically given him the why and left it up to him (within reason) to determine what he will chose to do. i can't make him read carefully or bring his books home from school. but if he doesn't like the grades he gets, or the consequences at home (ie less or no allowance, or maybe additional homework that we give him, or extra chores or whatever), then he will have to learn to change his ways.
i have also found that making sure he has the tools to keep himself on task is something i can do to help him. in previous years i've made a checklist and hung it up by the bookbags... these are the things you have to have done each day, have you done them? also, we try to make sure he has quiet, undesterbed homework space (not always easy to do with 2 younger siblings, but better than it has been in years past). And the biggest thing I find for me to be able to help him best is that i have to be focused on him and not preoccupied doing 3 or 4 things at once.
we have also found that once he gets into the routine of doing a few straight successful nights of homework, he likes the feeling of accomplishment it gives him, and that has been his own motivator. for example, he waited until LAST minute to pull together a report that was due. I made it clear that I was VERY unhappy with his time management, that I would help him this time, but that in the future he would have to be sure to do his work on time or face the consequences. We worked together for about 7 straight hours

dizzy) and got it done. He got a very good grade on the paper. In focusing on him, teaching him HOW to approach his big assignments, helping him walk through it, and coming out on the other end with a good grade made a huge impact on him. The next report that he had due was finished a week and a half early. And he is already working on his next report that is due at the end of October. He LIKED the feeling that he had done something well, and he took it and ran with it.
sheesh! that was a lot of rambling, sorry! summarized:
1) give the WHY rather than just demanding certain behavior,
2) give them a bit of freedom to mess up and learn from their mistakes,
3) give your focused time to help teach them how to have a succesfull homework session