i spend my whole weekend hassling, bribing, pleading, begging and eventually yelling at them to clean their mess up!
i hear you totally!!!
this weekend they have been 'working' on cleaning their rooms and they are still not done

the only way i have found to cut mess down is to have awesome storage for their toys or only let them use a certain amount of toys at a time
use to work well when they were younger say up till 5 years old but now - blah no hope!
My DD is unbelieveable- she keeps every piece of paper she cuts/uses , she keeps tags off clothes and her mess is sprawled everywhere- mind you she has our old bedroom which is full of cupboards and storage and she has stuff everywhere- the other day i found her nighty in a new take-away container!!!!!
i think they just get so overwhelmed with their mess that they dont know where to start or how to clean it up again
the mess just appears!!
sometimes it is heaps quicker for me to get in there with them and 'help ' them get stuff sorted- a PITA for me but stops the yelling

hugs hun wish i had a magic answer for you but i dont!!!