The word "heart" in Proverbs 4:23 is from the Hebrew [leb], applied "very widely for the feelings, the will, and even the intellect" (Strong's). The whole section of Proverbs 4 addresses this:

"My son, pay attention to what I say: listen closely to my words. Do not let them out of your sight, keep them within your heart; for they are life to those who find them and health to a man's body. Above all else, guard your heart, for it is the well-spring of life. Put away perversity from your mouth; keep corrupt talk far from your lips. Let your eyes look straight ahead, fix your gaze directly before you. Make level paths for your feet and take only the ways that are firm. Do not swerve to the right or the left; keep your foot from evil" (Proverbs 4:20-27).

Consider the same wisdom in the old children's song:

"Oh, be careful little eyes what you see; oh, be careful little eyes what you see;

 For the Father up above is looking down with love,

 So be careful little eyes what you see."

That verse is followed by, "Be careful little ears what you hear;" then "Be careful little lips what you say"; and "Be careful little feet where you go." It's amazing what wisdom was taught in Sunday School. 

The proverb tells us that the heart [feelings, will and intellect] is the "well-spring" of life."  "Understanding is the well-spring of life to those who have it" (Prov. 16:22); and "The words of a man's mouth are deep waters, but the well-spring of wisdom is a bubbling brook" (Prov. 18:4). The "well-spring" is the source of water, the source of life, literally and figuratively. It is refreshing and attractive. I remember a cabin in Pigeon Forge where I stayed with the senior class one year on our trip. There was a large screened porch on the back of the cabin, and a lively, bubbling brook ran within feet of it. It was the best place in the cabin. I even slept out on the screened porch a couple of times just to be near that sound. That's the way the wisdom of God should affect us, making our words peaceful and inviting, not dangerous "deep waters" that can be threatening and harmful."The power of life and death is in the tongue" (Prov. 18:21).

Jesus said, "What goes into a man's mouth does not make him unclean, but what comes out of his mouth, that is what makes him unclean" (Matt. 15:11); Jesus also said, "Out of the abundance of [your feelings, will and intellect, your] mouth speaks.The good man brings good things out of the good stored up in him, and the evil man brings evil things out of the evil stored up in him. But I tell you that men will have to give account on the day of judgement for every careless word they have spoken. For by your words you will be acquitted and by your words you will be condemned" (Matt. 12:34-37).

Putting evil thoughts from our minds is definitely a struggle, but God has given us His weapon against evil thoughts--His sword of the spirit--His word. "For the word of God is living and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart. Nothing in all creation is hidden from God's sight. Everything is uncovered and laid bare before the eyes of  Him to whom we must give account" (Hebrews 4:12). How can we fight bad thoughts, words, desires, ambitions? With the word of God. If it is your desire to overcome the evil within, keep a Bible everywhere you go, and when those things start creeping in, as soon as it occurs to you that they are present, pick up your Sword and just begin reading. And for the moments when you just can't have your Sword with you, have a few key verses memorized to fight with. I use "Create in me a clean heart, O Lord, and renew a right spirit within me."  God is never far from those who seek His will and way. 

"Let your gentleness be evident to all. The Lord is near. Do not be anxious about anything, but in everything, by prayer and petition, with thanksgiving, present your requests to God. And the peace that passes all understanding will guard your hearts and your minds in Christ Jesus" (Philippians 4:5-7).