The imagination training I am referring to is the very same development of the imagination that practitioners of direct-magic go through.
Also, while I'm sure occultists have found other ways to explore projection using dreamwork/trancing/whatever else, I'd like to clarify that the method I was referring to in my original post makes no use of such things.
It works the exact same way that a practitioner of direct magic would perform any normal feat of magic.
Here, assuming you had all the necessary skills I laid out, you would simply sit or lie down, then use your powerfully-refined imagination to operate projection directly, without any other kind of aid or the use of altered states of consciousness.
It is a straightforward, relatively instantaneous method.
But it is only possible
if you have thoroughly mastered the fundamental skill of the imagination and the transference of the consciousness.
Are you referring to the development of your imagination in this sentence?
If so, then there may be deeper issues slowing your progress.
Developing each imaginary sense shouldn't take any more than a few months each at the most - if this isn't the case, then that may be a sign that there are deficiencies in your mental development keeping you from properly progressing.
On the other hand, it could also be your technique in practice.
I would recommend exploring your imaginary senses and seeing if there is one particular sense you find easier than the rest.
If so, it would be wise to develop that sense so that it gives you a good idea of how to develop the rest of your senses.
In Rawn Clark's Bardon Companion, he mentions that he met a student that had been stuck on the Visual sense for years with no progress, but Rawn noticed that the student had a natural talent for the auditory sense.
He instructed the student to hold off on Visualization, and instead focus on developing his auditory sense instead.
The student found that he was able to master the auditory sense in less than two weeks, and doing so give him a deeper understanding of the development of the imagination as a whole, and he went on to master the rest of the individual senses in the weeks that followed.
If you find that there isn't really any particular sense that sticks out to you, and you wish to continue with visualization, then you may wish to examine your technique.
Personally speaking, I wasn't able to make any progress in visualization until I approached it from a very methodical, incremental perspective.
For nearly a decade, I had fruitlessly practiced nearly every day for a half hour or more each session, but recently I had made a change to my technique in practice that finally started to get me somewhere.
Beforehand, I would merely sit down with a few objects in front of me and try to visualize each of them in a generic fashion until I got tired - which of course got me nowhere after all this time.
But recently, I have begun approaching every object with a very structured pursuit of progress:
First, I take one object, give it a good look and try to remember a reasonable degree of its details, like normal.
But then, instead of simply trying to visualize the object in a generic fashion for the next five minutes or so, I start very very small, with very concise technique.
I close my eyes and imagine the object to the best of my ability, then solely focus on sustaining this static image while counting down from a predetermined duration. The visual fidelity is not important here in the beginning, all that matters is that I hold on to this conjured image with all my might for the duration of the countdown.
At first, I only started with 5 seconds.
If I succeeded, I would move on to another object. If not, I would repeat it until I did. This would continue until I had completed about 10 or so minutes of total practice, or I became tired.
In this perspective, every few seconds gained with the visualization of an object is a wonderful success. I started with only 5 seconds, but very soon I found that I could succeed with 10 seconds, then 20, then 30, and here I am currently with a full minute of success with any object. The fidelity of the image has also gradually improved automatically.
In only a week, I had gotten further with visualization than I had in a decade prior. And considering the fact that 5 minutes of success is the recommended standard for the completion of this particular practice, it seems likely that I will achieve that in the near future at the current pace.
Now, if you find that even with good technique that you cannot develop the imagination, then there may be a deeper mental deficiency that can only be remedied by taking a step back from the development of the imagination, and swinging back to your mental development.
The exercises in question are the various thought-control exercises which develop control over the fundamental consciousness, which I'm sure I have explained to you before.
Thought Control/Mindfulness Meditation, Thought Discipline/Focal Meditation, and Thought Vacancy/Void Meditation.
Before I recently made the aforementioned change to my practice of visualization, I had spent the last few months prior refreshing my mental development because I found that it had regressed somewhat, and I'm sure that my decision to remedy this weakness is a factor in my present success with visualization as well.
In my original post, I stated that Mental Projection is a lofty brick that lies upon the bricks underneath, namely the Transference of Consciousness exercise and the development of the imagination - but these bricks which support the Mental Projection above it also have their own bricks supporting them underneath, namely the Mental exercises/Thought Control.
This is why good courses of initiation like the IIH have you start from ground zero with Mental Development.
All the magical skills that occultists are so desperate for are skills which lie upon other skills....... which then also lie upon other skills.

You can't build a good house without a good foundation, and you can't be a good practitioner without a solid mental foundation and all the bricks between it and the higher skills you are reaching for.
Otherwise, practitioners are in for a rough and sluggish path of magic.