Conduct a series of body warm-ups as soon as practice begins. These warm-ups should involve shoulder and neck rolls, and possibly back massages.
Conduct a series of light vocal and breath warm-ups. One great exercise is to have everyone sing the highest pitch they possibly can, and decend in a slow glissando (like a fire engine) down to the lowest note of their natural range.
Play a C major chord on the piano, and have the choir sing do-mi-sol-mi-do on the syllable "Ah."
Repeat the previous step, this time playing and singing a C# major chord. Keep repeating, ascending by a half-step each time up to G major, then back down.
Number everyone off in groups of 4, such that 1s are sitting between 4s and 2s, 2s are sitting between 1s and 3s, and so on.
Instruct the 1s and 3s to sing C, while the 2s and 4s sing G, forming an interval of a perfect 5th. Hold the pitches until it sounds absolutely brilliant; every single choir member must be able to sing a 5th apart from either of their neighbors.
Conduct the 1s in singing the major scale from do to do, then back down again. Use 4/4 time at a very slow tempo, one beat per note.
Instruct the 2s to begin on do after 2 beats, as the 1s are singing mi. 3s and 4s should then enter likewise. At this point everyone should be singing do to do and back down, over and over, forming recognizable chords as they go. Signal them to stop after a few rounds, or when the harmony starts breaking down.
Conduct the entire choir in singing the major scale, from do to do and partially back down. Have the 1s stop on ti, the 2s on sol, the 3s on mi, and the 1s on do, such that a major 7 chord is formed. Hold the chord for 4 slow beats and rest.
Repeat the above exercise, except this time have the 4s stop on te, the 3s on sol, the 2s on me, and the 1s on do, such that a minor 7 chord is formed. Hold the chord for 4 slow beats and rest. Note that mi has been lowered to me, and ti has been lowered to te.
Repeat the above exercises for any chords in your choir's repertoire that could be improved. Each piece of music should be learned on solfege as well as the written lyric.