Setting your mind is like setting a thermostat. It is creating a target for the climate. Once you set a thermostat, the heating and air-conditioning will have to adjust in relation to the weather. It is a constant process, but the goal is for the system to create a life-giving climate. So too it is with our minds. Many people try to tell themselves to stop thinking negative thoughts -- which immediately brings to mind the very thoughts they are supposed to stop thinking.
“Set your mind on things above, not on earthly things.” (Colossians 3:2)
“Those who live in accordance with the Spirit have their minds set on what the Spirit desires.” (Romans 8:5)
-- John Ortberg in The Me I Want To Be