The first task is to get the shapes that are meant to be in different places within the illusory space to separate. Any ambiguity regarding the relative location of the shapes undermines the illusion.
How many of these variables need to be involved to get an effective separation? Let's look at a couple of images with an eye toward how the illusion is created, and how it might be enhanced.
It's pretty easy to see that the pointy rock in the center is closer to our viewpoint than the group of shapes in the upper part of the page. There is a little ambiguity on the upper left edge of the point. Why is that section confusing? Is it necessary to increase the differences there? If not, why not? If so, what can be done with color, value, edges or composition to turn up the separation.
The space is clear enough in the foreground area of this scene, but it gets a bit vague in the middle distance. Which variables could be adjusted to clarify where each of the shapes is located?
This is a well-composed image, but a little tweaking could make it stronger. Would color changes improve the feeling of space? How about value? Is it possible for shapes to be too separate from each other?
For homework, choose one of these or use your own image and adjust the dials up or down to make the feeling of space more effective.