Here is Cennino d'Andrea Cennini on the subject:
Now you must forge ahead again, so that you may pursue the course of this theory. You have made your tinted papers; the next thing is to draw.
You should adopt this method. Having first practiced drawing for a while as I have taught you above – that is, on a little panel – take pains and pleasure in constantly copying the best things which you can find done by the hand of great masters. And if you are in a place where great masters have been, so much the better for you [Ha! Thank you industrial capitalism for cheap, high quality colour reproductions!].
But I give you this advice: take care to select the best one every time, and the one who has the greatest reputation [this bit only holds good if the reputation-making process is sound]. And if you go on from day to day, it will be against nature if you do not get some of his style and of his spirit.
For if you undertake to copy after one master today and after another one tomorrow, you will not acquire the style of either one or the other, and you will inevitably, through enthusiasm, become capricious, because each style will be distracting your mind. You will try to work in this man's way today, and in the other's tomorrow, and so you will not get either of them right.
If you follow the course of one man through constant practice, your intelligence will have to be crude indeed for you not to get some nourishment from it. Then you will find, if nature has granted you any imagination at all, that you will eventually acquire a style individual to yourself, and it cannot help being good – because your hand and your mind, being always accustomed to gathering flowers, would ill know how to pluck thorns. [Emphasis added.]