As we prepared ourselves for our gathering (where we examined Psalm 20), we encouraged our church to spend some time reading over and trying to understand the main theme and rhythm of Psalm 18.
Why? 
Well, the reason for doing so is that David, in Psalm 18, reflects upon how God has delivered him from all of his enemies. 
Then, in Psalm 20, David builds upon what he has already said and turns to ask God to deliver King Jesus in similar ways. 
Thus, the themes of these two Psalms are tied together under the banner of deliverance for the King, with David asking God to do for King Jesus (Psalm 20) what God did for him (Psalm 18). 
As such, Psalm 20 is often referred to as a Royal Psalm; sung about the coming promised King who would fulfill the promise of God given to David in 2 Samuel 7. 
Therefore, it is a song of faith on the day of trouble, remembering how God saved David and pointing to how Jesus would be delivered on His day of greatest trouble. Through it all, we see how God never fails to do all that He has promised.