I recently attended a seminar on how to work with different skill levels and instruct others properly. A great word of advice that stuck with me was, "Some people just suck and you have to work with them and around them." The goal of coaching positively is to use the "sandwich technique" 80% of the time while analyzing, coaching and directing people that are struggling. When I first heard about this 'sandwich', I was not feeling it. It takes up too much time, comes across as fake if I have nothing genuine to say, plus it's just sugar-coating a problem. After testing the sandwich technique, I started seeing results that changed my perspective forever; I saw consistent long term success, happy infectious energy and higher self esteem.
When critiquing a singer, always make time to tell them a positive statement first and last. Cram in the advice and edits in the middle of the statement. For example, "Lauren, awesome diction. Everyone, let's make sure we match her perfect diction, also Lauren don't belt measure 12 because it's coming across as flat & heavy voweled, your head voice is powerful and bright so aim the sound there instead." The old me would of said, "Lauren, you're super pitchy and it's throwing everybody off, try it again." The honest factual truth is that Lauren is pitchy when she sings. The problem with bluntly and harshly blurting out the truth is that it's too general, comes across as a barked command, doesn't provide a proper example and is un-motivating. Taking the unfriendly approach also leaves a sour taste in the singer's mouth, increases negativity and lowers group morale. There are exceptions to this sandwich rule. I've found that a dash of military style get's the point across quickly. If a singer is repeatedly goofing off, unprepared, not blending, late to practice or just plain sucky it's not a bad thing to be critical because it can give them a reality check.
If your a cappella group has a weak link, give that person a few tries to nail her parts. "Lauren, you're singing the intro section rhythm wrong, here is the correct rhythm. Let me demonstrate that again. Now you try it 3 times in a row to lock it in your memory." Another approach is to ask singers to verbally repeat exactly what they need to change. If I tell multiple singers to each fix different things, most likely they won't remember all the notes. Try saying, "Before we sing it again, everyone repeat back to me what you are going to work on this time around so we get it right." Making them voice it out loud and put it in their own words, it provides them control and responsibility.
My last tip is to only accept singers in your group that handle direction and constructive critiques well. During auditions and call-backs give them a challenge, "Great solo! Just for the sake of testing vocal ability, can you sing that solo again but this time do it with no vibrato, taller vowels and and faster tempo." If a singer then blurts out excuses and gets defensive, then you should beware! Singers should have the capability to hear direction, accept it and apply it. Singers must also understand if I tell Lauren to fix measure 12 it does not mean I hate her, she's a loser and her hair is frizzy. All it means is that I know she has the potential to sing measure 12 better than she's previously shown.