Philippians 4:8:

Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.

See also:

Peace in a Storm
Dealing with Fear
How NOT to Deal with the Devil
Can I Command the Devil to Take His Hands off Someone Else?
Should I Use Meditation to Be Healthier?
Should I Use Positive Thinking to Be Healthier?
Are Suicidal Thoughts Evidence That Someone Is Demonized?
Objection: Mental Illness Is Treatable but Not Curable
Mistake: Fear