Visualization is one of the most important skills to master when it comes to magick, yet many have said that this is one of their greatest challenges. I hope to offer you a bit of help through my own insights here. Don't give up because you cannot do it as of yet. As with anything, practice, work and patience will get you there.
In Wicca, we use visualization to see the portals to the elemental realms, the elementals that stand at them, and the circle as it is cast around us. Witches use it in practically all magicks that we work. We visualize our desired goal, empower it with the energy we gather from the earth, the moon, the planets and stars, and direct it into a channel for it to manifest on the physical plane. Without skill at visualization, we'd have a hard time getting much of anything done. Visualization is essential to the proper working of the psychic senses. It's an essential skill for any witch or magician to know.
In our circle, we say "as above, so below." That statement has been used by magicians ever since the days of Hermes Trismegistus. Anything that you expect to have in the physical world, you have to see in your mind's eye first. Even in the most mundane of tasks, this is true. A builder doesn't just get a bunch of boards and nails and start hammering them together and expect to get a nice house out of it. She starts with a plan, a blueprint. Even an expert chef, who might not follow a recipe, can see in his head the end result of what he wants to make before mixing the ingredients. The point of visualization is to create something on the "above" so that it may manifest on the "below." Create it in the spiritual realm, the realm of possibility unmanifest, and the magick sets the wheels in motion so that it may manifest in the physical realm.
You can start developing your ability to visualize by working with known objects. To get good at visualization, you must first get good at paying attention to your own senses. If you aren't in tune with your senses in the below, how can they work well in the above? It's easy to get lost in one's own head, and to be so wrapped up in our thoughts that we don't pay attention to our bodies. The point of this exercise is to get you back in tune with your senses, to learn to work with them more effectively on a plane that you are familiar with - the physical plane, so that you can extend that familiarity to a less familiar plane - the astral plane.
Perhaps you have been there ten years,
Perhaps you have been there one hundred years.
Perhaps you are a precocious child,
or perhaps you are a wise great-great-great-grandmother.
Your roots reach down into the rich deep soil,
perpetually nursed by the breast of the Mother.
My feet leave the ground every few seconds,
but you never lose your connection,
not even for a step.