Is your child's room messy, due to lack of organization? If there are several toys and items that don;t really have an exact spot or are mixed up with others, your child may simply be confused. A lack of structure in the way the room is set up can lead to a child feeling discouraged and indecisive during clean-up time. Set up an organizational system that your child can easily follow. Labeling containers with pictures and words for each type of item is helpful for younger children. Older children should be allowed to set up their organization system themselves.
Can your children tell where things go in their room? This falls in line with the organizational system, as well as brings up another factor. If there is a designated clean-up spot for everything, it makes it less daunting. You know how you feel when you see that big pile of stuff. Imagine that same feeling from a child's perspective. Not having an organizational plan can stress kids further because not only do they have to pick up everything, they have to find a spot for it too.
Has your child been taught about value? Perhaps your kids throw things on the floor because they don't understand about value. If you buy them toys, books, clothing, and other items too often, those things no longer have value. It just becomes the normal thing to do. Thus, the items are just something that can be tossed around and replaced regularly. Set limits on the number of items you purchase, how much you spend, and more. Be sure the kids know these limits and are allowed to observe not only the spending part, but the earning process it takes to get the amount needed for each of their items.
Is your cleaning routine too drab? Perhaps your kids don't want to clean because it's too much of a chore. Just because it has to be done doesn't mean it can't be fun. Put on some music and dance the room clean. Give the kids each a reusable grocery bag or a kid's shopping cart, have them fill it up with items, and race to put them away. A creative and fun game will get their attention faster than screaming in frustration. It will also save you and the kids from further stress.
Is there more stuff than space to put it in? If your child has more toys than places to keep them, the problem is not your child being messy. The problem is too much stuff. What do you expect him to do with it if there's nowhere to put it? This calls for a sort and donate situation. You and your child can discuss kids that don't have enough toys to play with and how there is too much in his room. Together, go through the items and decide how much to keep, based on room to store it.