In general I think you need to practice a few things, namely:
-The basic forms
If you learn how to draw and render the basic forms, how to light them, how they interract with each other, you've probably all you really need. Combined with an understanding of perspective and you can place any of those forms anywhere in space. Reduce what you want to draw to basic forms and half your work is done. That gun is an elongated box, that leg is a cylinder, that spaceship is an egg, etc...
For the figure, I think you need a solid grasp of anatomy. Understanding that you can think of the leg as a cylinder is important, but you won't know to think of the knee as a box when viewed from position X without knowing how the knee is constructed and how it reacts when placed in certain positions.
At least...that's my idea of what you need to know. Maybe someone who's actually halfway decent at drawing from imagination will chime in too.
"Every generation sees the past though the lens of its own time." - Thom Hartmann