When you need to draw from imagination, you are also actively relying on your reference bank/memory of things you have studied and drawn before, when you have studied and drawn them enough, you will be able to (hopefully) rely more on free sketching/drawing from your imagination to draw these things, be it the human body and its range of movement, or scenery. Rather than overly depend on reference all the time to start an illustration.
To build up your internal memory bank of things to draw, there are some exercises I have learned from Kimon Nicolades’ The Natural Way to Draw that helps to do just that. To build the connections for your brain to remember how does one draw a certain thing. Going contrary to the book’s method of proceeding chapter by chapter and slowly being introduced to these concepts. I will just introduce them here directly briefly. Your success with them might vary and one can easily say its because you are not following the book chapter by chapter but we all know art isn’t as easily defined like that. These exercises of course should be supplemented with a lot of gesture drawings as well.
Exercise #1: Daily Composition: This exercise is deemed as the most important of the book, to be done daily, and its basically a searching gesture sketch of a scene from your head, and it doesn’t have to look like anything to anyone except you or even you, but it will get better as you do it more.
If you find yourself unable to think of a scene, study a everyday scene, like people queuing at a cashier, people sitting together at a restaurant table, remember these kinds of scenes as best as you can, and put it to paper at home as best as you can. Eventually, move onto scenes from your imagination. Let your pencil go free and do not worry overmuch over the details too much. Get your pencil moving and try to get what is in your mind onto paper. Do this as many times as you can.
Exercise #2: Reverse Poses: If you are unable to break away from a reference, you can try drawing the reverse of it, or a mirror version, it can be a lot harder than you think and stimulates the mind to start inventing with a base point of reference. Bring up your reference of a pose or anything really and just try drawing it the other way around or from another angle entirely. A good reference bank means being able to draw something from every angle from one angle of looking at it. Drawing a falling box (hence drawing it from many angles) is another example of this theory/exercise in action without a point of reference.
Exercise #3: Flash Poses: In a live model application, this will be if the model you are drawing runs up to the podium, takes a quick pose, a flash pose, then leaves the podium, leaving you with just an impression of what the pose was no matter how fast you were at looking at it. You have to work off this impression to draw. Without a model a quick look at a pose someone is doing in a photo and looking away/covering it up and drawing it, can work as well. This is meant to be a gesture drawing and not the full rendering but a gesture drawing can easily progress to that if need be.
Exercise #4: Descriptive Poses: Write down or read a description of a pose for example, a man sitting with his legs crossed and his hands are on his knees, his back is straight and he’s sitting stiffly. Try drawing a gesture drawing of this as best as you can then find a picture to countercheck if you wish or pose yourself and see how it looks.
Exercise #5: Memory Poses: The figure drawing studio application would be to have the model take say 5 poses, throughout these 5 poses, you do not put your pencil to paper yet but try to remember as much as you can, the detail of each of the 5 poses, try not to develop a memorization system to remember each pose in order, but try your best to remember the impressions of them in general. Then draw the 5 poses as best as you can, its possible to even hit past 5 poses and remember what poses they were and draw them. For a non-studio setting, study a series of pictures of someone taking different poses and then do the same.
In conclusion, there you have it, at least 5 exercises off the top of my head to try and see if your reference bank builds up more, it may seem hard at first, staring at a blank paper, but it will be worth it, to be able to draw more freely from your mind and build up the reference bank in your head.