Let the subject, at least in part, be the present moment, and because you are writing in that moment, the subject becomes writing.
Invariably, this leads to low-stakes writing or perhaps private writing because you won't likely be showing this piece to a reader-critic: unless you opt to enclose your present experience as a writer inside the amber of publication.
When I was younger and pretty much stuck in a prolonged block, I gravitated toward poets who mentioned the act of writing within their work. For instance, Octavio Paz's lines half-way down in his great long poem, "The river":
In mid-poem a great helplessness overtakes me, everything abandons me,
there is no one beside me, not even those eyes that gaze from behind me at what I write,
no one behind or in front of me, the pen mutinies, there is neither beginning
nor end nor even a wall to leap,
the poem is a deserted esplanade.
These meta moments inside texts helped by basically exiling future critical audiences and high-stakes situations from the moment. (At the end of my second MFA program and in desperate search of a full-time teaching job, I also used Post-Its with the single word "Buddha" or "Present" as exits into the Now--though I did not formally meditate at the time. I didn't own a laptop and would go to the basement of a university computer lab to type up my resume and cover letters, sticking these Post-Its onto the borrowed monitor.) In my most isolated stretches as a writer, I turned to metalanguage in my poems as a way to finally break free of audience-in-the-head and write; later on, the increased awareness of the present afforded by including metalanguage allowed me to feel the abundance of possibility and relationship that is writing.
In my writing journals, nowadays I include notes about process right alongside ideas and phrases for poems and essays. If I am working on scholarly writing, I jot down the date and time, some details about my writing area, (ideally, screened-in back porch as in right now, the rain mumbling, the eggplant half-fruit, half-blossom), and my private feelings about the project at hand. During any given writing session, I value those process jottings as much as I value the ideas or phrases. It's all writing--no sorting or categories needed.
"Yoga for Hands" is one way to turn to the experience of actually writing as subject material. (See post from 9/11/2012.) Another way to practice meta-mindfulness is to do a 7-10 minute freewrite on any topic (you chose) but every 2 or 3 sentences pause and, in parenthesis, pull yourself back to the Now. Write about the Now. Include details about the space you're writing in, sensory information from that space, the smell or appearance of ink, the sound of your typing, the heat given off by the keyboard, etc. Then return to what's outside the parenthesis, to the topic of your freewrite. Repeat.