Yes the most important principle being Draw what you see, not what you know. For example, don't draw an eye as an ellipse containing a circle, but look at the actual combination of of light and dark shapes, and draw those instead.
[Edit]. Also, drawing people is a really good test. If you draw a house or a tree, say, you can get the shapes / proportions wrong but the thing might still look plausible. Draw a head out of proportion, though, and it will immediately look wrong.
Non correct. Understanding of the form is a must. Drawing what you know is the basic principle. If you draw what you see without understanding of perspective, basic forms etc you will end with optically build representation of reality.
In case of the eye you must know how anatomically is constructed and draw by combining your knowledge with what you see.
[Edit]. Also, drawing people is a really good test. If you draw a house or a tree, say, you can get the shapes / proportions wrong but the thing might still look plausible. Draw a head out of proportion, though, and it will immediately look wrong.