(from Dundalk Md Charivari, Tom Wheatley, editor)
Singing a sustained note against a melody line that is descending. The sustained note will usually try to follow the descending part. The mental concept of moving upward against the descending scale will correct this.
Violent or abrupt changes from pp to ff or vice versa may affect the pitch.
Maintaining an accurate pitch is an ideal to hold before the singers you are coaching. Express proper gratification when it is achieved, providing the sing is satisfactory in other aspects. An occasional and moderate departure from true pitch is not a catastrophe. It is inherent in the amateur barbershop singer. It will happen, hopefully as the exception rather than the rule, so always try to remind your quartets and choruses that it's very rare that an audience or judging panel will react positively to a performance that is flat or out of tune.
Since poor intonation can be heard by most listeners and since there are so many reasons for a singer to either flat, sharp or otherwise deviate from the tonal center, a good and responsible coach will keep this subject in focus as a priority. He will become intimate with concepts and techniques of in-tune singing.