Please note that the developing voice in young people needs a very different type of exercise than the more mature voice. I am going to focus on the maintenance type of warm up for mature non-professional singers here.
1. Why Bother? Some of your singers may not have sung all week. If you just start singing, they could be straining those vocal chords. Body muscles need to be stretched before lots of activity. Well so do your vocal chords, diaphragm, and body.
2. Assume the Position. There are so many great ideas out there to get your singers ready to sing. Firstly, you must stand correctly with the weight on the balls of the feet that are slightly apart. Feel that the shoulders, hips and knees are aligned but not tense. Hands loosely at the sides is ideal. Chin should be down slightly with the shoulders not hunched. It should feel as if there is a string coming through the top of your head with your body all nice and loose but tall around it.
Now you can do some of these exercises. These are simple and fun.
3. Have Fun While Learning - I love to use rounds like Three Blind Mice or Row, Row, Row Your Boat. These help with hearing harmonies, being independent and enunciation. You try singing the last 2 lines of Three Blind Mice and see how easy it is.
Simple songs that have octaves or arpeggios etc. helps you to focus on what you will be working on in the songs in your rehearsal.
One of my favourite ways to warm up to to sing Twinkle Twinkle Little Star in staccato, or legato or in jazz or country style. Use a simple song and get your singers to make it sound like the style you are currently learning. I have even been known to have one half sing legato while the others sing staccato or operatic and folk styles. If you make your singers think and warm up the voices while teaching or reviewing musical qualities you make the process effective and memorable.
When we only practise every other week for an hour and a half, we are careful to use that time wisely. If you spend too little time on warm ups your sound may suffer. If you spend too much time, you may never get to learning the songs. Find that happy medium with the emphasis on HAPPY!!