G.H. Kersten, In the Footsteps of our Fathers, p. 52:
Bear in mind that Selah emphasizes that which is said and simply tells us to stop and consider, to pay attention to what was said.
Matthew Henry on Ps. 3.2:
Some refer it to the music with which, in David's time, the psalms were sung; others to the sense, and that it is a note commanding a solemn pause. Selah—Mark that, or, "Stop there, and consider a little." As here, they say, There is no help for him in God, Selah. "Take time for such a thought as this.
Cotton Mather, Psalterium Americanum (1718), pp. xvi-xvii:
The Word, SELAH, Ever[y] now and then occurs in the PSALMS. And now allow me to propose, every SELAH, as an Invitation for thee to make a pause, upon the Magnificence of thy SAVIOUR. I will not here make a long Recital, of the various, and very many Interpretations, which the Critics have left that Word in the dark withal. It shall be enough to say, That in the large, Thesaurus Philologico-Theologicus lately Publlished, there is a Dissertation upon that Word, which would perswade us, That SELAH is a proper Name of the Blessed GOD our SAVIOUR, and that it signifies, The Magnificent One, or, One that is exalted on high. The Word must not be too much feparated from the rest of the Discourse, but it is to be look'd upon as a part of it. It belongs to the Period, and is to be rendred, sometimes in the Nominative, sometimes in the Genitive Case, and sometimes in other Cafes; but most commonly in the Vocative. Christian, In the PSALMS every where think on thy SAVIOUR. And if a SELAH stop thee, think, before thou go on, O my SAVIOUR, How Magnificent art thou! -- Indeed, such is the Fellowship ever between our SAVIOUR and His People, that the condition of the One in the PSALMS will not forbid, but invite us to consider the condition of the Other. And therefore, what is now propounded, about a Contemplation of our SAVIOUR, to be a principal stroke of our Devotions in Singing of the PSALMS, will not supersede the Entertainments which are to be joined with it.
Reader, Make a Pause upon every Verse, and see what Lessons of Piety are to be learnt from every Clause. Turn the Lessons into Prayers; and send the Prayers up unto the Heavens: As Arrows from the hand of a mighty man, send them with lively Ejaculations up unto the Heavens.