What Does It Mean to "Bless God"?

A SONG OF ASCENTS.
Come, bless the LORD, all you servants of the LORD,
   who stand by night in the house of the LORD!
Lift up your hands to the holy place
   and bless the LORD!
May the LORD bless you from Zion,
   he who made heaven and earth!
— Psalm 134

Commenting on Psalm 134 in his book, Journey to Joy: The Psalms of Ascent (published by Crossway Books), Josh Moody writes about what it means for a human being to bless God:

How can an inferior person, a subject, bless a superior person, a king? How can a created person, a human, bless his or her creator, the God of heaven and earth? Scholars have attempted various solutions to this conundrum, because the idea of our being able to bless God does not merely occur in this psalm but is fairly frequent throughout the Old Testament. Psalm 72: 18 says, 'Blessed be the LORD, the God of Israel'; and Genesis 24: 27 says, 'Blessed be the LORD, the God of my master Abraham, who has not forsaken his steadfast love and his faithfulness toward my master.' How can this be? How can it be true not only that 'blessed be Abram by God Most High' (Gen. 14: 19), but also 'blessed be God Most High' (Gen. 14: 20)? Or how can we not only receive blessing from God but actually give blessing to God?

Perhaps we need to ask another question: what does it actually mean to be blessed? In English the word bless means to pronounce that something is good or to confer goodness upon something in a religious sense, and the word may have its origins in the Old English word blood. A benediction, frequently used as a synonym, means 'a good saying,' coming from the Latin root meaning to say that something is good or well. The Hebrew word used here for 'blessed' may have the root of meaning to kneel before something or someone, though not all agree with that derivation.

Perhaps it is simplest to say, by analogy, that this word blessed, often used of us blessing God and of God blessing us, functions similarly to when we say that we speak to God and that God speaks to us. When we speak to God , we are speaking, and we speak human words necessarily. When we bless God, we are blessing and give human blessing necessarily. When God speaks, he speaks God's words, and when he blesses, he gives God's blessings. So the blessing of God by humans is a human declaration that God is good. What the pilgrims here are urging the priests to do ('Come, bless the LORD') and what they themselves will do ('lift up your hands to the holy place and bless the LORD!') is to live a life, to utter words and do deeds, in such a way that makes clear that God is good. They are being urged to live a life that honors God, to live a life that focuses upon God, to live for God. They are being urged to say that God is good, that he is blessed. They are not adding to the divine, eternal, complete, sufficient blessedness of God in his own person; they are witnessing to it. They are declaring, in their own experience, through their journey, that they have witnessed that a life lived for God is the happiest kind of life. They are blessing God that he is blessed and worth living for. It is their witness, their declaration.