"Be joyful always; pray continually; give thanks in all circumstances, for this is God's will for you in Christ Jesus."
-1 Thessalonians 5:16-18 NIV
The notes say for each verse (NIV):
5:16 "People are naturally happy on some occasions, but the Christian's joy is not dependent on circumstances. It comes from what Christ has done, and it is constant.
5:17 "For the practice of continual (or regular) prayer."
5:18 "As in v. 16, Christians are differentiated from the natural man. Becuase of what God has done, they are continually thankful whatever the circumstances."