I've kinda avoided writing this for quite a while because it's a difficult subject.
It's a difficult subject because, even though the techniques involved are few and not that complex, it's describing them well enough so that people can understand and apply them, much less convincing people to persist with them until they are habitual. You'll see why as I go through them.
PART I was playing along with someone with a really good tone. It's the most effective thing that you can to.
If you listen and experiment and try to imitate the other player, you will do the things that I'm covering later on automatically, without being told to do them or how to do them, and without having to think about them.
Even playing along with recordings will help. If you can't often play with another player who has a good sound, at least listen to and play along with recordings of good players, then you can resort to the stuff in Part II.
PART II
Here are a few suggestions that I got over the years from my teachers:
Open your throat, almost as if you're yawning.
Blow like you're trying to drill a hole in the floor.
Don't pinch. Think of your embouchure as a rubber band around the mouthpiece with even pressure at top, bottom, and sides, not a clamp.
Here are some of the things that I teach in addition to those (the underlined stuff is the most important):
Blow so that that it seems that you're concentrating your breath at the tip of the mouthpiece.
Keep your embouchure stable, never "chewing" on it or changing it throughout the entire scale of the clarinet except for the extreme altissimo (especially never "reaching" for a note that you're unsure of by clamping down).
Hold your clarinet at the best angle (as seen in a previous article).
Take the right amount of mouthpiece in your mouth (as seen in a previous article).
Play a reed that's neither too soft nor too hard (the subject of a subsequent article and a note later on). Don't whine that "it's too hard to blow." It's not too hard. If you've been playing on soft reeds, you just aren't used to feeling resistance.
Blow so that you always feel some pressure at the back of your throat, even if you're playing softly. (The pressure you feel when you blow up a balloon.)
I also encourage students to play in the bathroom or some other place that bounces their sound back to them, and try to fill every corner of the room with sound.
That's all there is to it.
But many teachers talk about other things, like changing tongue position, "hot" and "cold" air, changing the "speed of the air" (which I'm pretty sure is impossible), changing hard palate position (which is physically impossible), "breathing from the diaphragm" (which is physically impossible), and on and on.
One very famous teacher actually had students play with toothpicks inserted in their embouchure at various places for reasons that never made any sense to me. (I wondered, if they developed their embouchure with the toothpicks, if they could they play as well without them!)
Another famous teacher did stuff like stuffing popsicle sticks in students' mouths at various angles.
Without going into the details of why I think these techniques are mostly useless and others simply can't be done, they are not necessary because, with the very few and simple suggestions above, plus experimentation and persistence, you don't need to do too much messing around.
Developing a good sound is neither THAT hard, nor THAT complicated!
Aside:
You can't develop a good sound if your equipment won't let you! Assuming that your clarinet itself isn't leaking or otherwise "broken":
If your mouthpiece is total garbage, as most mouthpieces that come with clarinets are, you'll never get anywhere. You don't have to have an expensive mouthpiece, but it at least must be decent.
If you're playing a reed that's too soft or too hard, your sound will either be too thin and your pitch will be hard to control. If you're playing a reed that's too hard, your sound will be stuffy.
If you're setting up your mouthpiece, reed, and ligature poorly, all kinds of nasty things will keep you from sounding well.
(By the way, don't expect your sound to transform from poor to fantastic by changing clarinets or buying an expensive pro horn. The difference in sound quality between a $6000 clarinet and a student clarinet is about 5%, if you need a number, and that 5% doesn't happen unless you're blowing them well.)
ANYWAY
At this point I have a choice between writing a book or trying to distill everything into stuff to do, and forget the whys. I'm going with the first option.
Do these things all at the same time and all the time, and use the suggestions above as a supplements:
Play with a stable embouchure, even all around like a ribber band, and don't pinch and don't mess with your embouchure across the registers.
Play with good breath support by opening up your oral cavity and putting pressure on the reed so that you can feel it at the back of your throat.
Play a decent mouthpiece.
Play with an appropriate strength reed.
A 3.5 is a good one to start with for most people from early teens through adulthood. Only the best players can control a reed and make a good sound on anything much softer.
Folks, there's no reason why you can't develop a good good sound. None at all.
All you need to do is apply some basic principles and do so consistently until they become habits.
There may be more that will come to me at 3 a.m., but this is enough for now. I'm already suffering over my marginal composition right now anyway and repeated myself too often.
Adios.
B.