Creativity is a muscle, much in the same vein that muscles are... well, muscles. In fact, there are a bunch of things I can liken to muscles. Intelligence is a muscle. Skill is a muscle. And they all need to work out to stay in working order.
Perhaps the biggest reminder of this to me is intelligence. In my high-school years, I was far from being the best of the best, but I was a regular honor-roll recipient — smart enough to mingle with the formidable dangers of the Pythagorean Theorem and long division. And now, less than a decade removed from school, I have trouble with even the most simple of maths. Of course, I have a calculator to help me with that, but often the process of making maths happen is the biggest hurdle.
I'd imagine the same is true for design. I've got the tools at the ready to solve any given problem, but not regularly exercising the process of design will inevitably hamper creativity. Much like my brain muscles get cramped up these days when I try the maths, after 3 years of designing ads for a local alternative news weekly, self-employment has been a bit of a challenge. My workflow has changed considerably during this transition, and not for the better. Where I used to find my days filled with designing things I didn't enjoy doing, at least I was designing. Nowadays, I spend much of my time trying to run a business — sending emails, replying to emails, making phone calls, losing bids on jobs, etc. — and I can noticeably feel my creative muscles weakening. While the end result is still great work I am proud to present to clients, the process takes longer than it used to. Which ends up resulting in many late nights finishing projects, or often starting over from scratch as the clock hands tick past midnight and into the one o'clock hour.