In general, I find that drawing from imagination becomes easier with practice and with drawing experience.
My approach to drawing something from my head begins (as with many artists) with a rough sketch. The lines of it might not indicate an actual object in the scene, but the general movement and position of those objects. I try to get as much of the image out of my head as I can relay, and then I look for references.
Occassionally, I'll do this in reverse, starting with a reference that looks a little like what I have in mind. I might begin copying what I need from it(such as the pose or the lighting) and then take off with it in my own direction.
As for advice, it depends on what you are trying to draw.
Is it scenery? Learn about perspective. You can establish a complex scene with a fairly simple grasp of perspective. Do quick sketches of city scenes or landscapes. Work big - by that I mean start with the big stuff and work down to the details.
Is it animals?
Practice, practice, practice! Don't draw just from your head. Spend time copying whatever references you can find - photos, drawings, live animals if you can get them to stay still! If you had a book on drawing when you were very young, it probably showed you how to draw animals out of circles and squares. The next step up from that is to make those shapes three-dimensional. Work on drawing the simplest of 3D shapes. I'm not joking - draw speheres, cylinders, cubes, eggs and cones. The next step is to combine these shapes to make a basis for your intended subject.
If you want to draw people, the same basic rules apply. You'll need some understanding of perspective to get complicated poses, especially those with odd camera angles like the work of
Hyung-Tae Kim (though it should be noted that he intentionally exaggerates his figures for effect). There are a lot of systems in place to construct a human head without looking at one. Check out these:
1 2
Again, start big and work towards the details. This is especially important for work from imagination. In general, it's best not to start with one feature, but to begin with the shape of the head. This way, everything is kept in porportion. An eye does not exist on its own; the size and shape of it are relative to the other features of the face, so you should plan out all of the features before you begin refining them.
I really recommend books by Loomis and Vilppu. These are hard to come by, but both teach great systems for drawing with and without models.
References can be found everywhere. Every serious artist has a collection of reference images. Your own body can be used as reference material, and there are programs like
Daz Studio (free, by the way) that enable you to pose a 3D model in a position you desire. You can then base your drawing off of that.
It is a misconception that artists draw everything straight from their mind's eye. A trained artists learns by copying what they see, and drawing without a subject takes a lt of time. Even the
masters made studies of models. The best artists never stop studying. They don't wake up one day and accept that they are "good enough", because that concept does not exist. There is always something to learn. I don't care if you could draw like Michelangelo from your mind, there's always more to learn.
This goes for manga artists, too. Many use idol photos as reference material. Get around enough and you might even find a drawing that you have seen the reference photo for(I have).
Using references does not make you any less of an artist! On the contrary, it will strengthen your work. All practice will help you to draw better from your imagination.
