During my drawing classes in Amsterdam, I give students the option to learn to create portraits. This is usually an important segment of a drawing lessons program and must be covered fairly extensively.
A good portrait consists of many layers of knowledge which come together to create great resemblance as well as a good piece of art.
As with everything else, good drawing is the basis, and once this is established, and a strong composition is found, the chances of a successful piece are greatly enhanced. Amsterdam offers a great option of small ateliers which bring live models in, for art students, professionals and hobbyists alike to draw and paint any way they like.
For aspiring artists who want to learn to draw the portrait, this should be a regular part of their artistic practice. However, lots of practice and a 'feel' for how to capture someone's likeness can never compete with strong knowledge of the anatomy of the skull, and a solid method of how to capture it. This is something I strongly emphasize during my Amsterdam drawing lessons.
The reason is simple. If you look at portraits made by amateurs, and even by the masters of the 17th century, you usually find that the face looks round and smooth. While this was highly appreciated back in those days, and will make you feel like you can draw a portrait, we have since then, found out that as it happens, it is better to draw certain areas as flat. A combination of roundness and angular sections makes for a far stronger statement on learning to draw a portrait.
During drawing classes, the only way to achieve such a result, is to have clear and solid knowledge of the planes of the head. This brings excellence of drawing and clarity of structure to the head you're drawing. You're no longer drawing the outline of a head and putting the features in it. You are instead drawing the volume of a skull, with its particular structure, and once you've captured it, the features fall into place almost automatically.
This is an approach that John Singer Sargent, the great master advocated, and which only started to make sense to me once I learned that, when drawing a portrait, you must assess the underlying structure in order to succeed. Features move and change, but the skull remains exactly the same, and this is what you're after.
During my Amsterdam drawing lessons I make use of great tools such as the Loomis method in order to bring structure to this understanding.
So in conclusion, lots of practice is very necessary to be a good portrait artist, but all the practice in the world will not get you to the peak of your abilities if it's not founded on sound knowledge.
Better to put in the practice hours while you are solidly informed of what you're doing than to blindly try to strike luck. Not doing this, you will risk being someone who after many years of attempting portraits, still runs into simple mistakes of alignment and proportion, and who's lukewarm art is surpassed by the crisp and beautiful art of a knowledgeable counterpart.