"Preserve my soul; for I am holy: O thou my God, save thy servant that trusteth in thee" (Psalm 86:2).
In "Biblical Perspective" Gordon Snider writes:
God has promised to preserve His people (Isa. 49:8). Often in the Psalms God is praised for this quality. Here David asked for God to be his guard and watchman so that his life would be saved. In previous lessons we have focused upon the faithfulness (steadfast love) of God to those who are in a covenant relationship with Him. The word here is the same; only this time it is used of man's faithfulness to his side of the covenant. The claim of the psalmist was not a boast of superiority, but a plea of sincerity. It was a plea based upon his covenant relationship with Yahweh. It might say, “Lord, I am one of your children. I have done what you asked me to do!” Save . . . that trusteth in thee. Here again David gave God a motivational statement for answering his prayer. He claimed a unique relationship between those who trust in each other.
Source: Studies in the Psalms: Adult Teacher's Insights, page 53.