Proverbs 4:23 (NIV)
23 Above all else, guard your heart,
for everything you do flows from it.
2 Corinthians 10:3-5 (NIV)
3 For though we live in the world, we do not wage war as the world does. 4 The weapons we fight with are not the weapons of the world. On the contrary, they have divine power to demolish strongholds. 5 We demolish arguments and every pretension that sets itself up against the knowledge of God, and we take captive every thought to make it obedient to Christ.
Philippians 4:4-9 (NIV)
4 Rejoice in the Lord always. I will say it again: Rejoice! 5 Let your gentleness be evident to all. The Lord is near. 6 Do not be anxious about anything, but in every situation, by prayer and petition, with thanksgiving, present your requests to God. 7 And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus.
8 Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things. 9 Whatever you have learned or received or heard from me, or seen in me—put it into practice. And the God of peace will be with you.