
Draw something from reference. Let's say it's a skull.
Hide the reference and draw it again from memory. Focus on essential planes and basic forms, not little surface details.
You will be imediately confronted by what you don't know about it. Is the mastoid process above or below the teeth? What is directly above the last molars? Things like that.
Write your questions down.
When you use terms like "above" and "below", make sure your questions are phrased in terms of the imaginary axes of the object itself -- not the axes of the paper. If you ask questions like "In this particular view, is the base of the nose above the base of the back of the skull?" you will be memorizing a drawing -- not an object's form. The idea is not to memorize a series of drawings, but to learn the object itself, so that later you can use what you learned to draw it from any angle. In the photo above, the blue lines are not a fixed grid in paper-space; they are the edges of cubes. When you turn the skull off axis, mentally turn the cubes off axis as well.
Draw it again from reference. Answer your questions.
Repeat the process with and without reference until you have fewer and fewer questions and can draw the object well without reference.
Move on to another view of it and repeat the process until you don't need reference for any view at all.
You may be surprised at how fast you can develop a "conception of form" for a complex object.
The feedback question / answer loop is better than staring at an object and trying to memorize it through sheer force of will.
The nineteenth century French artist Horace Lecoq de Boisbaudran was a pioneering proponent of this general sort of learning method.