I struggled with this for a loooong time. I finally found what worked for my 11 year old. I told him I would help him thouroghly clean his room. We went through everything. He got rid of some stuff he didn't use anymore. We found a place for all his things. Organized everything.
Then I said "When you get ready for bed make sure everything is put away. If it's not I will donate it or sell it on Ebay and I keep the cash! I don't care what it is. It will be gone."
Two days later he left pokemon cards scattered on the floor...they were sold on ebay.
He now knows I'm serious. His room has been clean since January! 