Sing unto him a new song; play skilfully with a loud noise.
Sing ... a new song - the song of redemption from all foes, and from the great adversary, especially at the crowning consummation (Psalms 40:3; Psalms 96:1; Psalms 98:1; Revelation 5:9, "They sung a new song ... Thou art worthy ... for thou wast slain, and hast redeemed us to God by thy blood out of every kindred," etc.; 14:1,3; 42:9-10). Whilst the song of redemption is of ancient date, it is ever new to the heart of the believer (1 John 2:7-8).
Play skilfully. The same Hebrew ( heeyTiybuw (H3190) nageen (H5059)) is used by Saul, 1 Samuel 16:17, "Provide me now a man that can play well" - literally, 'do good to play.' Saul's words impressed themselves on David's memory. An undesigned coincidence, and a proof of genuineness.