I think it means, like Psalms 119-11, tells us, to hide the word of God in our hearts, that we might not sin against Thee. We should sing hymns and spiritual songs to ourselves (instead of all the other garbage we hear on the radios.
When the enemy hits us, we are already prepared with the word of God in us.
David sang, danced, payed, all the time. 

First I'm kinda new at this but I think it is along the lines of 1 Thessalonians 5:17. "pray without ceasing" This would be one way to do that. Sing to yourself about how great God is, how merciful he is, and thanking him.