This is a tip I learned at a Relief Society Super Saturday during a class on TEACHING YOUR CHILDREN TO WORK given by a beautiful lady named Anne Pratt. Her suggestion was to tell the kids that in five minutes, the MESS-UP MONSTER was going to come and eat everything that was laying where it didn't belong.
This monster can be made of a box or laundry basket with a posterboard cut-out of a monster taped to the front, such as the one above. (I googled "monster art".) The toys, clothes or other items this monster "eats" (i.e. you swiftly going through the room depositing the items in the basket, preferably making monster noises) have to be put in storage for a week, a month, or longer, whatever you deem appropriate, or they have to be earned back by doing extra chores or giving up allowance. This idea has worked wonders for me. It's effective, fun and SCARY!