It is very important to take posture into account in playing any instrument. Playing the clarinet we have to be careful with the body posicion, the strenght we apply with our fingers, our mouth posicion and respiration.

Body posicion
When playing, our head has to be looking ahead, in a straight angle in relation to our body. Legs have to be a bit sepparated and the body has to be straight, though in a confortable position. The clarinet should form a 30-45┬║ angle in relation to our body.

In this example, the position is quite correct, though the shoulders would need to be a bit back and the clarinet should be lower.

Fingers
The weight of the instrument is held by our right thumb (left if you play a lefties intrument) and our mouth. The most strenght we apply with our fingers when playing, the most weight the thumb has to hold. It is a good idea to play in front of a mirror to see if we apply too much strenght (if we see the tips of our fingers turning yellow when playing or the rings of the instrument are marked on our fingers, we are applying far too much strenght). The clarinet will play by only convering the holes with our fingers. We should also not raise our fingers too much, it will cause a greater effort for us to play faster.

Mouth
To play properly, the upper jaw teeth have to rest on the mouthpiece (we should really buy a mouthpiece saver) and the reed has to rest on the lower lip, which covers the lower jaw teeth. We will use the upper lip to close our mouth and make sure no air gets out. When we play, the lower lip has to be tense as if we were forcing a smile, and we have to watch out not to store air in our cheeks, because sound and air pressure will be affected. You sould not apply any strenght with your mouth, only make sure no air gets out. It is really important to have the lower lip tense, otherwise the sound will be poor.

Breathing
A good breathing technique is essential to play the clarinet. We use diaphragmatic breathing, which is the one we use when we sleep. The idea would be to lower the diaphragm (that is, to inflate your belly withouth moving your chest) when we inhale and to move the diaphragm up (belly in) when we exhale. Our belly would be like a balloon, full of air when we have inhaled, and losing air when we start blowing into the clarinet. A good way to work that is to lay on a hard surface, with your back straight and a weight on your belly (a book or so). If the book moves up when we are inhaling, things are working.
You should also work pressure and direction of the air flow. An easy way to do that is to get a mouthpiece and attaching a balloon to the lower end, then trying to inflate the balloon without inflating our cheeks (we really need a mirror here). When we are able to do that, we put the reed on the mouthpiece and we try to produce a sound and inflate the baloon at the same time withouth inflating the cheeks. To make it easier on you, try to inflate the balloon a couple or three times before attaching it to the mouthpiece, and when you're able to do it properly, reduce the size of the balloon.
To work on direction, get a straw (the ones you'd find in a juice or a milkshake) and pretend to be playing the clarinet with proper breathing control.