The clarity of mind depends on many factors.
The clarity of present mind is affected by the clarity of previous mind.
It is also affected by the body.
The clarity of mind is also affected by the sila. Anyone who doesn't have clean way of life is far from perfecting the clarity of mind.
There are so many other factors.
For the food itself, it is very clear that there are direct relationship between what we eat and the clarity of mind. After eating, we will have difficulty to meditate. It is not possible, but it is very very challenging.
Onion is also affecting the clarity of mind. This is not something we have to accept by blind faith. But something you should try yourself. If you don't trust it, try to eat food without garlic for 2 weeks, afterwards you do a meditation. Don't be surprise if your mind is clearer. We argue about this also no use, this is something to be tried.
For the breathing itself, there are countless technique.
The clarity of mind is also affected by the breathing. There are like 1 to 1 relationship. If we can breath naturally after some time the mind will clear. This is why Buddha encourage do a meditation with breathing as the tool. Because they have 1 to 1 relationship.
In case, the monkey mind is really crazy and the clarity of mind is very difficult to obtain, you cannot do much. You can just let it be, until that monkey itself tired.
There is a special technique called holding the breath.
How if you do this simple experiment.
Notice the mind in 1 minute, do the thoughts running away here and there?
How long can you hold your breath and still feel comfortable? 30 seconds? 20 seconds?
Let's say you can do it for 30 seconds, holding your breath within comfortable zone
You hold it and notice your mind. You will notice that the thought that previously easy to move here and there, now it has difficulty to fly here and there.
Even if you want to, it is very difficult to change the thought.
I don't say when you are holding your breath, you cannot change the thought, but the though that previously has a tendency to fly here and there, now it is reversed. Its tendency is to stay still.
Again, this is not something to be argued and debate, but something to be tried and proof it by yourself.
This meditation technique can be useful to quickly kill the monkey when he is crazy. After, the monkey KO, the clarity of mind will be there.
I am not here nor there.
I am not right nor wrong.
I do not exist neither non-exist.
I am not I nor non-I.
I am not in samsara nor nirvana.
To All Buddhas, I bow down for the teaching of emptiness. Thank You!