The battle rages in our house between the have to dos and the want to dos and it can get nasty on occasion. I have a 9 year old son. I try to pick my battles and like janh, stand ny ground by threatening removal of nice things like gameboys and playing out till the have to do things like homework are done.
I think two things ignite the situation and it's got lots to do with my son's age, but not much to do with impending puberty.
On the one hand my ds is at an obsessive age. He's old enough to really get into things now - able to read well, concentrate for long periods, follow instructions, play out by himself etc. So we have crazes on pokemon, beyblades, cycling stunts, Harry Potter, skateboarding, fishing, ebay, game boy, cooking, x-box, etc etc. Each week or so, one craze comes to the fore, and it is all important and in my son's mind comes before anything else and fills his conversation. Set that against an increasingly busy school and extracurricular schedule - more homework, music practice (he's just taken up the tenor horn at school), cubs and a fossil collectors club at weekends etc and his week is much fuller than it was when he was in the infants. So he is under pressure to organise himself much more.
Partly as I result of advice on mumsnet, I have reduced his extra curricular stuff. He's stopped going to judo and swimming lessons (temporarily - he has a grommit). I feel that one activity a week after school is all he can manage if he is to fit in the have to dos in with the want to dos. It's made a big difference.
So, to add to all the good advice here helenmh, can you look at your son's routine and see if you can cut a few things out of it, so there's less pressure on his time?