Is their mess invading your space? Is there something about their lack of organization effecting you and your life? Or does it just bother you that they aren't organized? If it just bugs you, and isn't effecting you, than do nothing. Their life their space let it be. Now if their lack of organization is effecting you ASK them how you can help them get better organized so they don't need to rely on you? When they say I don't know, that is when you give suggestions.
There is not much you can do. Try getting a few baskets and label them. Like about 4 baskets. They will not immediately become attentive to detail (putting small things away properly) but they will throw something in a basket just to keep it off the floor. Tell them that by using the baskets it decreases the amount of time wasted on trying to find things. A broad category is better then none or the floor.