1. Foreground, middle ground and background
Depth is a key to make an environment coming alive. Used in the right way it´s able to pull the audience into the picture. Defining a foreground, middle ground and background creates depth.
2. Degree of details
Variation in degree of details can be used to create depth. As here using many details in the foreground and fewer in the background.
3. Depth of field
Depth of field creates depth. The farther away the more blurred.
4. Thick strokes
Thick strokes in the foreground and thinner strokes the farther away creates depth.
Perspective creates depth. Lines disappear into vanishing points and things or people becomes smaller the farther away they are.