Turning Heart

Deuteronomy 29:18
Make sure there is no man or woman, clan or tribe among you today whose heart turns away from the LORD our God to go and worship the gods of those nations; make sure there is no root among you that produces such bitter poison.

Moses is discussing the covenant God made with Israel. In the verse preceding this one, he reminds them of the gods of Egypt and Canaan, "detestable images and idols of wood and stone, of silver and gold", and he cautions the whole community to supervise each other's worship. One individual's worship of false gods can poison the whole nation, so they all were to be vigilant.
The heart can turn from right to wrong. Jesus knew the fickleness of the human heart. I don't think there are any magic formulas to guard the heart. I believe it is done a day, an hour at a time. Check the secret thoughts and desires, read the Word, study it, offer the sacrifice of praise and thanksgiving, learn from wise Christians, talk it over with God, repeat as necessary.