I am currently teaching some Art Lessons still and we are covering George Bridgeman's drawing techniques. Every aspiring artist needs to study and copy from this great master teacher.
Bridgeman had such a grasp on the strategy of a strong underlying drawing.  The principle to remember is that you can always soften a drawing, and it will soften.  It is harder to strengthen later on in the process.
Simplicity is key also.  Notice in these images from the Hand's book for example.  There are very few curved lines.  Draw from one point to the next and then change direction and make a relatively sharp transition. The transition will soften later.
There is so much to teaching drawing but that is a beginning.

Thank you for visiting and here's to the next post...
If you have questions or comments, email me at curebyart@gmail.com.