Romans 12:2:

And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.

See also:

Mistaken Prayer Mindsets
Dealing with Fear
No Big Deal: When Impossibility Meets Omnipotence
Quit Worrying About Your Health!
How to Deal with the Devil
Is “Seeing Yourself Healed” a New Age Visualization Technique?
Can Your Mind Heal You Without Jesus?
Should I Use Meditation to Be Healthier?
Should I Go to a Hypnotist to Get Free from Habits?
Should I Use Positive Thinking to Be Healthier?
Objection: “Faith Healing” Is Simply Mind over Matter.  Unbelievers Can and Do Get Healed Just by Having a Positive Mental Attitude, and It Has Nothing to Do with Jesus.