A foundational principle of music is relativity. Therefore, when it comes to amateur choirs and groups who learn harmony, there are two things that are crucial to singers' individual and collective success. (1) Singers have to get used to hearing parts that are different from their own, without being "pulled away." (2) During musical rests – especially long ones – singers must develop the ability to remember the sound of the previous note(s) they sang, so that they won't experience harmonic disorientation (lose their place “vertically”) while waiting to sing the next phrase.
The following exercises can help develop polyphonic independence and strengthen the “inner ear” of amateur choral singers. All exercises, whenever possible, should be fashioned from the music that is being taught.
RESISTING THE "PULL"
(1) Learn a sequence of chords out of rhythmic context. It’s easier for singers to process harmonic information, when they’re not under the “pressure of the metronome.” Once singers know their respective pitch sequences, adding the rhythm becomes easier. Speed = familiarity.
(2) Have the choir sing sustained chords. Some choir members need opportunities to get used to listening vertically, hearing their own parts along with others. The closer the pitches in a chord are, the stronger the mutual “gravity” between them. So pick a chord from the song on which you’re working, and have the singers hold it, on the first syllable of the word with which it occurs. Have them do this 3 to 7 times. You can also move the chord up and down a couple of half steps for variation.
(3) Have singers perform scales, arpeggios, and “spell” chords, based on the material at hand, to aid them in recognizing their particular orientation within the chord.
KEEPING ONE'S MELODIC PLACE
(1) Connect all melodic phrases, creating one big melody with no “gaps” (rests). A lot of notes are forgotten in the gaps. Research shows that people find it easier to remember groups of notes than individual pitches. This finding lends itself to the idea, that forgetting what note comes next is a sign of a failed intervallic connection, between the previous note that was sang, and the upcoming pitch. So connect them. Cut out the rests. The further your particular song’s arrangement deviates from the key, or the more disjunct and angular the interval “between phrases” becomes, the more valuable this exercise will be.
(2) Have singers practice remembering the sound of the pitch. The ability to hold on mentally to the sound of a pitch, after it no longer sounds, is called audiation, and it’s the foundation of aural skill. Play one note on an instrument. Call for total silence for at least 10 seconds. Have the choir sing the note back to you. Ask for a show of hands indicating who “lost” the sound. Repeat. As they get better at it, play unrelated chords and pitches during their silence to try and throw off their inner ear, providing them with more of a need to focus inwardly.
The more your singers become used to the harmonic sensation, and the stronger their inner ears become, the better their harmonic singing will become. Be patient, because people have different levels of musical aptitude in general, and harmony in particular is one of the most challenging aspects of singing. You neither want to discourage those less developed singers, nor stunt the growth of those who are more developed. Depending on the degree of skill diversity with which you're presented, you might mix up the repertoire with the various skill levels in mind, or if an option, compose pieces that play into the levels and strengths of your singers.