Philippians 4:8 (New International Version)
8Finally, brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things.
Am I commanded to just think positively? to be optimistic? Hmmm. I think that's too general and broad to help me with my thought life. Through Bruce(the shark from Finding Nemo) I realized that the first step to control my thought life is by stuffing my mind with God's Word. That I believe is crucial, if my mind is without the Word of God how will I expect it to glorify Christ? Philippians 4:8 is a great start. It teaches me the specific things I must dwell on to.
Romans 12:2 (New International Version)
2Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will is—his good, pleasing and perfect will.