I get many comments saying my work is so 3D, and has such depth. It's because of the shading, of course. The hatching (several line strokes going in the same direction) helps, but what really fools your brain into seeing depth, is the dark shading beneath or to the side of a shape.
With a realistic drawing, you need to place your shading very carefully, but with abstracts like zentangle, you have room to play. So if you think of this kind of shading as an enhancement--Threedy becomes that shy little dark area that peeps out from behind a shape. He likes to hide in corners. He usually picks a side and stays there throughout a whole drawing. Sometimes he chooses a side and either top or bottom.
The bigger he is, the more depth implied, but he's never more than a third the size of the shape he is shadowing--try not to let him get even that big very often.
Hope you find him helpful!