Construction is one of the most important aspect when learning how to draw the figure.
Not only it will help you to create more solid drawings when drawing from life at a life drawing classes or when sketching in the street but it will also allow you to draw from imagination without references.
Here is already a little gem that was recommended to me by Scott Eaton when I attended an anatomy training many years ago.
The following pages will show you how the entire body relates to the height of a head.
Those are obviously generic proportions and will not apply in all circumstances but this will be a very good starting point to use as reference when drawing from life.