The selection of microphones for any given situation can have a great impact on the challenges of feedback.
The two major ways that microphone selection can play into feedback is the directivity of a microphone, and the number of microphones you are using.
All microphones have a directivity pattern attribute. Some microphones are omnidirectional—they pick up sound from all directions.
These are usually terrible choices for live sound, as two of those directions are going to be from the loudspeakers and monitor speakers. Another is the cardioid pickup pattern. This pattern picks up sound well from the front of the microphone, but as you get further around from the front towards the back of the microphone, the amount of sound it will pick up from that direction decreases.
These are typically excellent choices for live sound needs, as the rear of the microphone tends to be the part that is directed towards the PA system and monitor speakers.
There are also hypercardioid and supercardioid patterns, which pick up less sound from the sides, but tend to pick up more sound from the rear compared to the basic cardioid patterns.
If you use monitor speakers, this will make it more likely you’d get feedback compared to a regular cardioid pattern as that’s the part of the mic pointed at the monitors.
Using a cardioid pattern mic for your stage microphones will likely give you the least trouble with feedback.
Another challenge with feedback is mic’ing choirs. A common mistake when the sound person is having a challenge picking up a choir is to add more microphones.
This often has the opposite effect, because each time you add another microphone to your mix you lower the overall volume you can achieve before getting feedback.
If you’re having a problem pickup up the choir, and let’s say you’re using six microphones, try using only two microphones and see what happens.
Also, give the choir the bare minimum they need in their monitor speakers, and definitely do not put the choir mics in the monitor speakers.