A shadow is the result of the direct relationship between the location of a subject and its relative position to the light. Most landscape photographers associate shadows with strong sidelight. But strong shadow images can be made when a subject is top lit. What comes to mind are circumstances such a walking underneath a boardwalk in the middle of the day, an old broken-down slotted roof with shadows cast on the floor or walls, or aerial perspectives of birds or other flighted subjects blanketing their shadows on the ground. The stronger the light, the deeper the shadow it creates. It stands to reason that the best shadow images using the sun as a light source are made on cloudless, pollution and haze-free days.
Shadows make great anchor points and compositional elements. In fact, if the shadow is to be the focus of the image, it should be a primary element of the composition. The introduction of a shadow into the framing of a photo immediately adds a dimension of creativity and makes an image more graphic. It adds depth and dimension to a photo, giving two-dimensional images a third dimension. It allows us to think we can reach into the photo and touch what appears in the foreground. This is powerful stuff.
Treat a shadow with as much respect as you would the primary subject. As I mentioned earlier, it may even be the primary subject. Use the Rule of Thirds when deciding where the shadow should be placed in the frame. Bias your exposure so the shadow density is rich and dark. Be careful to not underexpose or else you'll muddy up the other tones in the photo. Use Levels or Curves in Photoshop and work just the shadows to deepen the blacks. Additionally, be careful you don't blow out the bright areas on the highlighted side of the subject that creates the shadow. I suggest you bracket. It's better to be able to make an exposure decision when the images appear on the monitor, than to go back and do it again. If the sun is your light source, work early or late in the day as sidelight tends to produce the best shadows with warm light. If you intend to work with studio lights, position them and use the modeling lights to preview the shadow's intensity and distance. The next time you're out in the field, if you're not already a shadow shooter, take note of them and work them into your compositions.