Example has more followers than reason.

Arguments persuade the part of a person that argues back. Watching someone actually do the thing bypasses that entirely, because it settles the question of whether it can be done. This is why counsel from a person who does not practise it lands so poorly, and why quiet consistency changes rooms that speeches never reach.

Pick the behaviour you have been trying to talk someone into and do it visibly yourself for a month without commentary. If they shift, it will not feel to them like losing an argument. That is precisely why it holds.