How do you teach a 7 year old to pick up thangs after themselves?

success: Rewards and role modeling are the best ways to encourage a kid good habits. Paradoxical interventions (leaving his mess untouched until it bothers him) is an extreme measure that in some cases might work. Finding out the reasons why he does not do it spontaneously is advisable. Sometimes pushing does not work, if he notices too much interest, he might use it as a way to express himself negatively.

Encourage: Many children need a "reason" to cooperate. Give him encouragement- "when you pick up, we can leave to go to the store". Give him specific instructions- "let's put all the trash in here, then we'll put the clothes in the hamper". Give positive comments- "i like how you lined up the toys on the shelf. It looks great". It takes time, but they will start to see the benefit of their cooperation.