In the preface to his commentary on the Psalms, Matthew Henry has a helpful & instructive section on the proper use of the Psalms. There are many gems in this excerpt (the numbering is Henry’s though I have broken the paragraphs in places and added bold & italics).
All scripture, being given by inspiration of God, is profitable to convey divine light into our understandings; but this book is of singular use with that to convey divine life and power, and a holy warmth, into our affections. There is no one book of scripture that is more helpful to the devotions of the saints than this, and it has been so in all ages of the church, ever since it was written and the several parts of it were delivered to the chief musician for the service of the church.
1. It is of use to be sung. Further than David’s psalms we may go, but we need not, for hymns and spiritual songs. What the rules of the Hebrew metre were even the learned are not certain. But these psalms ought to be rendered according to the metre of every language, at least so as that they may be sung for the edification of the church. And methinks it is a great comfort to us, when we are singing David’s psalms, that we are offering the very same praises to God that were offered to him in the days of David and the other godly kings of Judah. So rich, so well made, are these divine poems, that they can never be exhausted, can never be worn thread-bare.
2. It is of use to be read and opened by the ministers of Christ, as containing great and excellent truths, and rules concerning good and evil. Our Lord Jesus expounded the psalms to his disciples, the gospel psalms, and opened their understandings (for he had the key of David) to understand them, Luke xxiv. 44.
3. It is of use to be read and meditated upon by all good people. It is a full fountain, out of which we may all be drawing water with joy. (1.) The Psalmist’s experiences are of great use for our direction, caution, and encouragement. … (2.) Even the Psalmist’s expressions too are of great use; and by them the Spirit helps our praying infirmities, because we know not what to pray for as we ought. In all our approaches to God, as well as in our first returns to God, we are directed to take with us words (Hos. xiv. 2), these word, words which the Holy Ghost teaches. If we make David’s psalms familiar to us, as we ought to do, whatever errand we have at the throne of grace, by way of confession, petition, or thanksgiving, we may thence be assisted in the delivery of it; whatever devout affection is working in us, holy desire or hope, sorrow or joy, we may there find apt words wherewith to clothe it, sound speech which cannot be condemned. It will be good to collect the most proper and lively expressions of devotion which we find here, and to methodize them, and reduce them to the several heads of prayer, that they may be the more ready to us. Or we may take sometimes one choice psalm and sometimes another, and pray it over, that is, enlarge upon each verse in our own thoughts, and offer up our meditations to God as they arise from the expressions we find there. … If, as St. Austin advises, we form our spirit by the affection of the psalm, we may then be sure of acceptance with God in using the language of it…”
The Psalms were thus serviceable to the Old-Testament church, but to us Christians they may be of more use than they could be to those who lived before the coming of Christ; for, as Moses’s sacrifices, so David’s songs, are expounded and made more intelligible by the gospel of Christ, which lets us within the veil; so that if to David’s prayers and praises we all St. Paul’s prayers in his epistles, and the new songs in the Revelation, we shall be thoroughly furnished for this good work; for the scripture, perfected, makes the man of God perfect.