Prov4:23- Guard your heart with all diligence, for out of it are the issues of life.
The seat of everything we do or think is the heart. When bible talks about the heart using small letter 'h', it is talking about our mind which controls most of our actions. The heart isn't referring to the blood-pumping organ in our chest.
If we can take care of what goes into our heart, then we have succesfully taken of care of our lives. The eye&ears are gates of the heart that needs to be secured, you must be careful about what you see&hear.
To mount guard over your heart, you have to give place for God's word at all times, read healthy book&feed your eyes&ears with only what is pure.
Action point
Father, I receive grace to mount guard over my heart In Jesus Name.