Little writing trick from Louise Erdrich (from The Paris Review, The Art of Fiction 208) :
I started writing poems with inner rhymes but as they became more complex they turned into narrative. I started telling stories in the poems. But the poems I could write jumping up from my desk or lying on the bed. Anywhere. At last, I had this epiphany. I wanted to write prose, and I understood that my real problem with writing was not that I couldn’t do it mentally. I couldn’t do it physically. I could not sit still. Literally, could not sit still. So I had to solve that. I used some long scarves to tie myself into my chair. I tied myself in with a pack of cigarettes on one side and coffee on the other, and when I instinctively bolted upright after a few minutes, I’d say, Oh, shit. I’m tied down. I’ve got to keep writing.
(Bear writing coaches work as well. Typing tied to chair in Google Images yields surprisingly few pictures of writing-related bondage.)