Some thoughts from "Desiring God" by John Piper on worship...
"What goes on in the heart when worship is not in vain? It is more than an act of mere willpower. All the outward acts of worship are performed by acts of will. But that does not make them authentic. The will can be present (for all kinds of reaons) while the heart is not truly engaged (or, as Jesus says, is "Far away"). The engagement of the heart in worship is the coming alive of the feelings and emotions and affections of the heart. Where feelings for God are dead, worship is dead...
Now what does this imply about the feast of worship? Surprisingly, it implies that worship is an end in itself. We do not eat the feast of worship as a means to anything else... Genuine feelings of the heart cannot be manufactured as stepping stones to something else... If you have been floating on a raft without water for three days after a shipwreck on the ocean, and there appears a speck of land on the horizon, you do not say, "Now to what end shall I feel desire for that land? What good end should now prompt me to decide to feel hope?" Even though the longing in your heart may give you the renewed strength to get to land, you do not perform the act of desire and hope and longing in order to get there.
The longing erupts from deep in your heart because of the tremendous value of water (and life!) on that land. It is not planned and performed (like the purchase of a plane ticket) as a means to getting what we desire. It rises spontaneously in the heart. It is not a decision made in order to... anything! As a genuine feeling of the heart it is an end in itself.
Or consider fear. If you are camping in the Boundary Waters of Minnesota, and waken in the night to the sound of snorting outside, and then see in the moonlight the silhouette of a huge bear coming toward your tent, you do not say, "Now to what end shall I feel fear?" You do not calculate the good results that might come from the adrenaline that fear produces, and then decide that fear would be an appropriate and helpful emotion to have. It is just there!
All genuine emotion is an end in itself. It is not conciously caused as a means to something else. This does not mean we cannot or should not seek to have certain feelings. We should and we can. We can put ourselves in situations where the feeling may more readily be kindled. We may indeed prize some of the results of these feelings as well as the feelings themselves. But in the moment of authentic emotion, the calculation vanishes. We are transported (perhaps only for seconds) above the reasoning work of the mind and we experience feeling without reference to logical or practical implications.
This is what keeps worship from being "in vain." Worship is authentic when affections for God arise in the heart as an end in themselves. In worship God is the dreaded voice on the phone. God is the island on the horizon. God is the bear and the setting sun and the "most favoristest" rocket and the mother who gave it and the big, strong fourth-grade brother.
If God's reality is displayed to us in his Word or his world, and we do not then feel in our heart any grief or longing or hope of fear or awe or joy or gratitude or confidence, then we may dutifully sing and pray and recite and gesture as much as we like, but it will not be real worship. We cannot honor God if our "heart is far from him."