Clients ask me everyday, "how can I prevent my voice from breaking into falsetto when I sing higher?" The short answer is "by mixing", the longer answer is "there are different approaches to mixing". It depends on whether you sing naturally in thick folds (aka chest voice) or thin folds (aka head voice), it also depends where the problem melody sits in relationship to your break as to whether you sing it in a 'head based mix' or 'chest based mix'.
Mixing is essentially the sound of heavy winging. It's the sound of Kevin (from Kevin & Perry Go Large) the teenager saying "oh mum it's not fair!" It's the irritating sound we (well some people) make when when we're in total meltdown that we can't have something. It could also be the sound of being really really joyful about something. Think about it, songs are written by songwriters when they are emotionally charged up and when they are composing those melodies they will be using that part of the voice which naturally conveys emotion. All we really need to do as singers to get it, is practice whining! This is easy to do in isolation and without sustaining a tune as it's simply a burst of sound. However, it requires a certain amount of athleticism to sustain in song. It can also sounds really awful at first hence the reason why a lot of people never master it and just give up. But I'm guess you're not one of those people!
From a mechanical point of view mixing happens at a pitch where the vocal folds need to start to thin out in order to vibrate faster for the higher pitches. If they thin out without having your stabilisers on, the back end of the vocal folds will flip up (in order to elongate), and the breath pressure will suddenly release causing a gap at the midline and a consequent breathy voice. When we apply the heavy winge we are tilting the thyroid cartilage forward (along with other movements). The thyroid cartilage houses vocal folds stretching them from the front maintaining sub glottic pressure (aka volume) and therefore stability. Finding this 'mix' requires a number of factors to be in place.............
1. You need active posture.
Your normal standing stance isn't enough. Practice standing with your back against a wall with your feet 6 or so inches away from the base of the wall. Make sure your bum is touching the wall, your upper back and the back of your head (if the back of your head doesn't touch the wall use a cushion or rolled up towel and prop it behind your head). This dynamic posture enables your support muscles to kick it to the best of their ability. Avoid jutting your chin forward too much as you start to sing. As little is ok, but you don't want the muscles at the back of the neck shortening too much. Also it helps to look upward a bit.
2. You need core strength.
Train those abs, back and neck! I advocate Pilates and Yoga and a posture called 'sleeping tiger' (google Dahn Yoga Sleeping Tiger). Strong muscles will enable your body to take the weight rather than your vocal chords.
3. Keep your tongue relaxed and forward.
While whinging your larynx will be trying to lift and to help it on it's way, keep your tongue relaxed and forward (see picture of Miss Jessie J).
4. Keep your false vocal folds out of the way.
Because this is a relatively high effort sound, sometimes you may be using too much energy. To ensure you don't block the free vibrations of the vocal folds keep them squidgy monsters (aka false vocal folds) out of the way. Hard to explain as it's more of a feely thing, but try to keep a sense of wideness in the vocal tract as you're performing these sounds. This one is a bit tricky because is requires two opposing force in one tiny place. On the one hand mixing requires effort at the point of the vocal folds but on the other hand, just above your vocal folds you want to acquire a sense of openness and freedom a bit similar to that of taking a silent breath and or just before you giggle. See me for a one 2 one it's much easier to get you to do some exercises than try to explain it.
4. It's a balancing act.
Too much breath pressure under the vocal folds and they will break open (aka flip), too little breath pressure under the vocal folds and they will break open too. Modify the width of your mouth to compensate. So, if you need to reduce some of your breath pressure (if you feel like your being strangled) widen the space between the lips when singing and if you need to create more breath pressure (if you're not able to create the umph you need) minimise the space between the lips when singing (like a ventriloquist).
5. Practice top down.
At first hit the challenging notes from the top down rather than trying to sing a scale up to them. This burst of sound is much easier. Do this on an ergh sound first, then experiment with oo, oh, ee, ah and aye.
The quieter you can sing these exercises the harder your support muscles will be working but the more control you will gain over it.
7. Keep the scales simple.
When you do finally decide the scale up to those challenging notes, please keep it simple. Don't try backflips up there yet. Use a slide or the first 3 or 5 notes of the major scale or the chromatic scale if you're more advanced.
8. Routine problem passages.
Now that you're used to going up there, take a problem passage in a song and practice it in a lower key working up to the original key or higher. You will find that the combination of the different vowels, consonants, rhythms and pitches will present way more challenges that simple scales. Every problem has a solution though including trying a more manageable song or changing the key.
Of course all these tips are much easier to grasp with me showing you how and guiding you every step of the way. I promise I don't bite!