A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 149:1This Psalm sustains a close connection with the foregoing. The chosen people are exhorted to praise God, in view of past favors, and also future victories over enemies, of which they are impliedly assured. (Psa 149:1-9)
(Compare Psa 96:1).
psa 149:2God had signalized His relation as a sovereign, in restoring them to their land.
psa 149:3in the dance-- (Psa 30:11). The dance is connected with other terms, expressive of the great joy of the occasion. The word may be rendered "lute," to which the other instruments are joined.
sing praises--or, sing and play.
psa 149:4taketh pleasure--literally, "accepts," alluding to acceptance of propitiatory offerings (compare Psa 147:11).
beautify, &c.--adorn the humble with faith, hope, joy, and peace.
psa 149:5in glory--the honorable condition to which they are raised.
upon their beds--once a place of mourning (Psa 6:6).
psa 149:6high praises--or, "deeds." They shall go forth as religious warriors, as once religious laborers (Neh 4:17).
psa 149:7The destruction of the incorrigibly wicked attends the propagation of God's truth, so that the military successes of the Jews, after the captivity, typified the triumphs of the Gospel.
psa 149:9the judgment written--either in God's decrees, or perhaps as in Deu 32:41-43.
this honour--that is, to be thus employed, will be an honorable service, to be assigned
his saints--or, godly ones (Psa 16:3).