SING ALL. See that you join with the congregation as frequently as you can. Let not a slight degree of weakness or weariness hinder you. If it is a cross to you, take it up, and you will find it a blessing.
SING LUSTILY, and with a good courage. Beware of singing as if you are half-dead or half-asleep; but lift up your voice with strength. Be no more afraid of your voice now, nor more ashamed of it being heard, than when you sing the songs of Satan.
SING MODESTLY. Do not bawl, so as to be heard above or distinct from the rest of the congregation— that you may not destroy the harmony—but strive to unite your voices together so as to make one clear melodious sound.
SING IN TIME. Whatever time is sung, be sure to keep with it. Do not run before nor stay behind it; but attend closely to the leading voices, and more therewith as exactly as you can; and take care not to sing too slowly. This drawling way naturally steals on all who are lazy; and it is high time to drive it out from among us, and sing our tunes as quick as we did at first.
ABOVE ALL, SING SPIRITUALLY. Have an eye to God in every word you sing. Aim at pleasing Him more than yourself or any other creature. In order to do this attend strictly to the sense of what you sing, and see that your heart is not carried away with the sound, but offered to God continually; so shall your singing be such as the Lord will approve of here, and reward you when He cometh in the clouds of heaven.
(The Works of John Wesley, Vol. XIV (Grand Rapids: Zondervan Publishing House), pg 346. Quoted in Kenneth W. Osbeck, The Ministry of Music (Grand Rapids: Kregel Resources), pg 61.)