It is most important to keep your thumbs and hands visible when talking to a class. Practice holding your hands out with the thumbs pointing upwards until you can do it naturally. This is a very powerful dominance gesture, although it is one which does not come naturally to everyone.
If you find this is too difficult to do naturally, then try holding your hands at your sides, slightly curled, with the thumbs visible. This has less effect but is still powerful.
Hand movements should be calm and slow.
When you are having to wait too long for silence try closing your eyes, while using the hand and thumb technique above, and counting visibly. The more observant children will Shhh the class for you.
It is a good idea to have a hot spot where you always stand to tell the children off. After a very short time the children will become quiet as soon as you go there. The hot spot should be reserved for this purpose.
These techniques are taken as evidence of self-confidence by the children.
It is essential that you expect good behaviour at all times. Your expectations comes across in your subconscious body language. If you do not expect good behaviour then you will never meet it.
You can use body language and careful speech patterns to defuse many situations. It is always better to avoid confrontation wherever possible without losing face. It is important that you allow children to avoid losing face too. A child will often back down if they can do so without friends seeing it. This is why taking a child outside the room for a few words usually works much better than shouting at him or her in the class.
If you have the time, then wait until the class has gone, sit down and talk to the errant child, perhaps with a desk between you while s/he is standing. This removes the need that the child has to look cool by slouching, because you are looking more cool by sitting down. The child sees you as confident and in control. It means that you do not have to tell him or her off for bad posture and keeps the two of you a safe distance apart, reducing the likelihood of a flare-up. The worst that can happen is that the child storms off, and that can be dealt with on a later occasion, perhaps by a senior colleague
A few experienced teachers may be scathing about the use of body language in class control, even though they subconsciously use it themselves. You should be cautious in discussing body language with colleagues because there will be some malicious teachers who will tell the children about the hot spot or raised thumbs techniques. Once the children are aware of what you are doing the techniques become much less effective.