Have a dedicated, clear, consistent place to leave your keys when home. Work too, if applicable.

Remove social media from their phones.

Agree, for many persons it won't be an easy thing to want to do but the how to do it is so easy that everyone could do it: long press the app and chose Delete/Remove. And it will do them so much good.

Clean your closet/house. Put everything into containers/boxes. Only take things out of the boxes when you need to use it. At 6 months everything still in a box you don't need.

• don't ever - if at all possible - buy on credit
• consider before buying anything whether you truly need it or only want it; also, what will the maintenance of this thing involve and what will other consequences be when owning it
• learn to be content with having enough; have a healthy view of "enough"
• Have a healthy routine in life (so do not commonly approach days weeks and years with "let's see how things go")
• speak with people who have successfully simplified their life
• try to have realistic expectations

...and so forth.

Have less stuff. Just straight up buy less and get rid of stuff that you don't need on a regular basis.

Only One?

• Only do 0.5 of any important things. Do more 'Useless things'.
• Eat food not junk views
• Of any two options, choose the third
• Be Kind to everyone and everything as much as possible
• Practice meditation, yoga, tai chi or similar
• Listen to music, better yet play an instrument
• Eat Real food. Home cooked. Naturally.