Before anyone gets too upset, just hear me out. I came from a fine art background so I am trying to be as unbiased as possible. I'll will let you be the judge of that.
Let me say this first.
So for the most part I have always been able to draw or use whatever is in hand to illustrate. With more years under my belt I have grown my skills and have gotten to a point where I feel comfortable being able to draw well...anything and do it well.
I believe this gives you a bit of an advantage when it comes to design. There seems to be no better way to understand the concept of form and space then trying to recreate that object on a plane. For example, being able to study how the human body is put together, how the skin sits on the muscles and gets wrinkled at curtains places and proper proportionality of anatomy.
All of these observations take place in the eye and brain. Taking mental notes trying to learn. But ask someone who cannot draw to visually show that information they just learned. Even if they spent an hour studying just the eye, they would only show a poor representation of what they had just studied.
Why? Do we forget what an eye looks like? Well of course not. But we have not practiced the forms. Many of us have to learn by doing, especially something that relies heavily on observation. We have to use our hand to practice laying down lines over and over again till we start to build whatever we are observing on the paper.
This repetitive practice of using these physical motor patterns and nerves firing help us learn these forms in a better fashion. That is why the vast majority of designers who have a background in illustration have awesome understanding of the basic gestalt theories already. Why? Because they have to, it they didn't then they would most likely not be very good illustrators.
Practice recreating forms and experimenting with space and volume. Teaching your eye not to just obverse but to be able to reproduce it down these physical pathways. Eyes have terrible memory so it is up to us to ingrain the motor pattern so that our brains are more in tune with this fine detail and level of observing.
I took a summer art class when I was younger and my teacher said,"Drawing is 90% observing." Which makes sense. Look carefully and thoroughly so you can properly understand the shape in front of you.
Draw more, people.