I can almost never turn off my mind. My brain is always thinking. It's part of the reason I wasn't very good at things like Guitar Hero or DDR (or DsDsR, the handheld homebrew Nintendo DS version).
It's the unconscious brain that's faster. When master athletes and singers and dancers are doing their thing, they aren't thinking anything - their minds really are a blank. In fact, thinking about it inhibits their ability to do what they're good at. The same is not true for beginners, which is perhaps why I am quick to pick up new things but slow to master them.
In any case, I have trouble not thinking. Once I realized that was part of the problem, I worked on it, and I got slightly better with the video games, but there's still thoughts hanging around in there that I'm trying to ignore. There are only a few things, then, that can clear my mind effectively.
The first one, which requires some effort, is meditation. I've refined my ability a little from practice at the beginning and end of every martial arts class I've taken for over 6 years. With that, I still get the occasional meta-meditational thought, but I've learned to turn them into water striders, skimming across the sphere of my mind without breaking the surface. I let them be, let them float away on their own rather than turning my attention to them and chasing them out.
My second imperfect option is music. It's easy enough to let the instrumentals wash over me and let the rhythm tap my foot for me, but if there are any words, they stick in my mind, recruiting all the associations my brain has made with the subjects over the course of my two decades, bringing back that jumble of conscious thought and unconscious qualia that normally inhabits my head.
The last two are the only ones that clear my brain completely. One, related to my first method, is the actual practice of martial arts. Doing my forms helps me focus on tasks. When I have to write papers or carry out some other arduous task and I find myself being sidetracked, I often take a break to practice my forms. The tunnel of focus built by the end of the set carries over to my work and helps me avoid distractions.
The other is the one that has worked since I was old enough to want that respite from thought every so often. It's the ocean, or in truth any other large body of water, like a lake. The intricate patterns are mesmerizing. The deep rich bass undertones of rising and falling swells; the constantly moving melody of shadows and glittering reflections effervescent and transformed by the intrusion of a breath of wind, a wave from an oblique direction, or the wake of a boat; the sudden punctuating appearance and crash of attention-grabbing whitecaps, the cymbals of the ocean's symphony.
I can simply stare for hours. It's amazing considering I can sit still that long even with some books (it really does depend on the book). But a simple Oriental rug-sized patch of water in a windy sea, or a still smaller area, whitened by the passage of a boat, holds enough complexity to keep my conscious and unconscious well occupied.