One clear focus of Jesus' teachings is the emphasis on being pure in heart. Among the blessings he proclaims, he singled out the meek or humble, those who hunger and thirst for righteousness, the merciful, the peacemakers -- all of which come from a pure heart -- and then singled out a special blessing for those who are pure in heart. He emphasizes being clean on the inside, and how being clean only outwardly is hypocrisy. Christians have long adopted the psalm "Create in me a clean heart, O God" as a regular prayer, part of a standard worship services in the liturgical churches, and often a favorite among the songs that may be sung at times in the non-liturgical churches.
But it is easier said than done. I have realized this summer that I have let the exhaustion of the long weeks at work take their toll on me, in becoming frustrated and resentful at the long hours and lack of time to do much of anything besides eat, sleep, and work. I think that's why "repent" is such a frequent refrain in Jesus' teachings. The call to repent went hand-in-hand with a call to be baptized: a washing that was more about the need for purification in our hearts than our bodies.
So how does God create in us a clean heart? Is there a medicine chest in scripture for that kind of thing, when we look up and realize we have let ourselves become sarcastic or bitter or resentful? Repentance is key: deciding to turn from wrong to right. Prayer is also helpful, particularly praying for the people I resent (whether I have a right to resent them or not) can be healing. But the more time I spend reading Scripture, the more benefit it seems that I get from it. Jesus told his disciples, "Now you are clean through the word which I have spoken to you" (John 15:3). I find that spending enough time reading Jesus' thoughts and actions does have a cleansing effect. I can feel the difference afterwards as some of the dirt on my soul is washed away.