Across the spectrum of human culture, from the Chinese opera soloist to the Muslim mu`aziin to the European choirboy, the singer's body is the instrument; and control of the instrument is the foundation of technique.
Contraction of the gluteal muscles is often said to help to counteract the tightened stomach muscles required for full use of the diaphragm. It can also prevent cases of bowback posture, a spinal contortion usually inade of singers trying to gain control over their full vital capacity, at the cost of their posture.
Another school of thought says that tightening the gluteal muscles is degenerative of good and full tone production. In this way of thinking, with the tightening of the gluteals comes a rigidity in the abdominal floor muscles and this precludes the so-called diaphragmatic muscles from being free to move which lessens the depth of tone and causes the diaphragmatic breathing to be limited to frontal movement whereas with a release of the gluteals, that movement can extend downwards into the pelvic cage (causing the butt to expand slightly) and backwards into the lower back (expanding the lower ribs and back). This method is more in keeping with Alexander Technique (a system of ergonomic body use that aids in vocal production).
Palate - Overview
"Raising the soft palate" (the squishy part of the roof of your mouth toward the back) creates a more resonant mouth and opens up the nasopharyngeal space for air to pass over the palate which is an important aspect of singing with your full natural range with no vocal breaks. There are two schools of thought on this (or maybe more). One is that the palate should be released, not so much forced up as high as possible until more skill and co-ordination have been attained. The second school favours a more lifted soft palate ideal.
To release the soft palate (it will release up)
Think of arching the eyebrows up, sometimes called 'bright eyes'. This can help activate the muscles responsible for a lifted palate, which are not recruited in daily activity. Another way is to imagine you are slowly inhaling to smell a yummy rose and want to get as much yummy roseness in your nose as possible.
The nasopharynx must be open (this is the connection between your throat and your nose and lies in back of your uvula and goes upwards into your head). If your nose is closed off as if you are swimming and don't want water to go up your nose, then your range will be somewhat limited on the upper end and you may use your throat muscles to aid your singing which can be a very big strain on the actual vocal cords.
In your mouth alone there are about a hundred things a pettifogger could come up with to tell you, but let's just stick to the more important ones. Your tongue should not have a groove in the middle but be rounded and resting gently against the bottom teeth (not pulled back from the teeth). A grooved tongue puts undue pressure downwards onto the larynx and prevents the cords from freely functioning and sometimes can cause flatness. You want your jaw to be loose and free to open and close as needed and not locked into a position, be it open or closed. If it is not, your tongue will become more active and this can put undue pressure on your larynx and prevent your cords form freely functioning.
A little tip to tell if you are on pitch is to cup your ear and listen to the note. It makes the sound resound in your ear enabling you to tell if you are on the same pitch.
Often singing is taught by saying that the singer must have their stomach muscles in action so that they can "sing from your diaphragm". They are further instructed that they must always sing from down low; that they will damage their throat and larynx and such if they try to force sing from the top.
However, this is confusing and ambiguous to many people and it is difficult to explain to someone how to "sing from their diaphragm". An alternative way to achieve this is to put into play good and tension-free posture with an emphasis on body ergonomics, the breathing will naturally come from the "diaphragm" (this is slang for meaning that the lower abdominal muscles and body are involved in the breathing). With "diaphragmatic support" comes a greater depth of tone, less inclination to have the singing unduly centered in the throat, a more even sounding voice, and a reduction or elimination in vocal breaks/cracks.
It's usually easiest to begin with your arms and hands at your side. Your ribs are actually quite important. They really have to be free and somewhat controlled in their contraction as you sing, otherwise you will sound breathy and your range will not be as high. However, this is beyond the scope of the skills of the lay person and it's best to pursue this with a vocal instructor.
Locking your knees is an absolute NO! If you do this, you can block air circulation and cause yourself to pass out. Believe me, this has happened a couple times in a few of my choirs. Locked knees can also cause pelvic tightness which, as we have discussed in the aptly-named "Butt" section, goes against what we want. Standing with your feet about shoulder width will also help you because it provides you with a better base, and if you are nervous on risers you are less likely to fall. Proper alignment can also help the breathing to be more full-bodied and tone to be fuller. One way this is taught sometimes is to put most of the weight onto the balls of your feet without lifting the heels from the floor. This helps again with posture and not locking the knees. A second way is to imagine your feet and sinking down, barefoot, in some thick pond mud. This may re-balance your body a bit and you may feel a sway as this happens.
Just as a human in peril can shout even when contorted, powerful and expressive singing is possible from almost any body position. The fundamentals of vocal performance are, however, generally easiest to master when the body is in what many trained vocalists refer to as a 'neutral' posture -- a state of relaxed readiness to support the whole-body effort of producing the best sound possible.
The commonest posture advice given to beginning voice students is to 'straighten the back' in order to be able to take deep breaths in and control their vocalized exhalation. Practice in front of a mirror is frequently advised, but not truly necessary. The eventual goal is for the singer to unconsciously adjust his or her posture to suit the demands of song, whatever external body position might be required.
The human spine has a natural curvature to it, with a concave lumbar and a convex thorax. Almost every human adult spine is warped from the theoretical ideal in some regard or another, a few to the extent that the condition interferes with good health and good singing. Even the best innate spinal curvature is, however, subject to numerous misalignments that result from imbalances amongst the various muscular pulls that hold the spine upright.
Despite the apparent simplicity of simply standing erect without hunching, leaning, tensing, or over-arching one's spine, a few moments' calm reflection whilst erect usually enables even seasoned professionals with years of singing experience to sense points of unevenness or over-tension. These disparities arise quite naturally in the body with changing states of mind, and are an important and ordinary part of unspoken human language. Eliminating them altogether is as daunting a challenge as standing truly immobile without rigidity; though feasible and sometimes desirable, it can also produce flat and lifeless song. The goals when starting out are less ambitious: Seek to know what your body is doing, and then begin exerting some intention over those active parts of your personal carriage that were heretofore unconscious.
One deceptively simple exercise in attaining what singers call a neutral posture is to imagine the spine lengthening as one stands. Refrain, as much as you can manage, from consciously trying to change your posture. If you can safely maintain your balance, it may even benefit you to stand with your eyes closed. Simply visualize your spine lengthening, as though your body were suspended from the top of your head instead of being supported from below. Fantastical self-suggestions that exceed the strict bounds of reality are often surprisingly helpful; try to feel your feet rooting to the ground or floor, your hips resting firm like the base of a great mountain, your head brushed by cool moist clouds high above. Taken further, this can become almost a mystical exercise in itself; a number of Asian calisthenic meditations employ similar methods. All considerations of spirituality aside, it is technically beneficial towards developing mindfulness of one's own stance.