Increase the quantity of your rehearsal time. You'll need quality rehearsals as well, but the amount of time that you dedicate to practicing is very important as well.
Provide copies of the music that you will be performing prior to rehearsals so that members can practice and learn the music.
Instruct band members to memorize their lyrics before they even come to practice. Stumbling over the lyrics can diminish the quality of your group's vocals and prevent them from being able to focus on worshipping God as they sing.
Isolate each person's vocals at rehearsals. Have each singer sing his part individually to make sure that the notes are accurate.
Tape rehearsals and performances and play the recordings back for your group's members. Listening to their singing can be a tremendous help in having them recognize areas that need to be strengthened.
Instruct members of your group to think about and meditate on the meaning of each song. If the singers really use the songs to praise and worship God, they will be able to sing with more conviction and emotion. This translates into better vocals.