Several years ago I lived with one of my best friends (after her divorce). We experienced a similar problem with the kids not picking up, not cleaning up etc. This is what we did that worked for us....it can be costly, but it worked. Every now and then (once a week or so) we would go through the house and pick up everything that belonged to the kids that was obviously not put away or where it should be...dump it all in in the middle of the living room, then tell the kids they had 15 minutes to get it picked up and put away where it belonged...or it was gone! This really worked well for us. We only had to do this a few times before we began to see results, and it's amazing the improvement it made when the kids lost a few of their treasures.
Naturally we didn't permanently get rid of some of the costlier items (but they didn't know that.) There were some things though that they did lose that really made an impact...and every now and then they had to re-do homework (rarely, but there was a few times.)
This really did help to get the kids to understand that we meant what we said. I wasn't their mom of course, but they had to respect me too since I was the responsible adult in the home at night (she worked nights).
