Here are 9 simple techniques that can help you create depth in your drawing to make it look more dimensional.
You can draw objects in such a way that they overlap one another to show that they exist on a different plane in space. By overlapping, you will be able to tell which object is in front or behind.
In this example, the lonely square on the right is not overlapping or being overlap. There's no way to tell whether it is in front or behind the other three squares.
Draw objects that are further away smaller than objects closer to you. That's how perspective behaves in real life.
Use thicker lines to bring objects to the foreground. This technique has got to do with perspective as well. Lines look thick when they are close will look thinner as they go further away.
Show more than one side to the object
Show the sides of the objects that you're drawing.
Use shadow to suggest form
You can use shadow to give more information to the object you are draw, especially the form of your object. A circle can turn into a sphere with shading. And when shading is added to the bottom of objects, it will appear that the object is laying on some surface.
Placement of objects relative to horizon
The eye level line is the horizon line. Things placed closer to the horizon line are further away. For example, the small square appears further from Square A because it is closer to the horizontal.
The small square and Square B are exactly the same distance away because their bottom line is the same distance away from the horizontal line. In this case, you can also tell that small square really is smaller than Square B. Whereas Square A, even though drawn at the same size as Square B, is bigger because it's closer in the foreground.
Objects in the background has less contrast than those closer
Objects in the foreground have more contrast, either in terms of line weight, value or colour saturation. This is how atmospheric perspective will affect how you see things in real life. Objects further away are blocked by dust particles in the air, hence their colours will be dulled down, and contrast reduced. The further the objects, the less contrast there will be.
When you draw, you can vary the line weight or value of your object.
Objects in the background has less detail than those closer
You can see closer objects in more detail than further objects. Draw using the same way.
Use hard and soft edges to draw or divert attention
When your eye focuses on an object, especially something in close distance, objects in the background will appear to be blurred out. You can use soft edges or transitions to mimic that effect.