I cured my children of this very same thing. Take a large box, put everything which is where is shouldn't be, write LOST PROPERTY on it, and put it in the shed, preferably in a 'tough to get to part'. Took mine a few weeks to get used to not leaving stuff, and gettting the standard answer 'have you tried lost property?' to 'muuuu'uuuumm, where's me shoes'