A few of my writing friends and I are on deadlines, which can sometimes be rather stressful, so we're egging each other on with a new (to us) technique. 1 kph...
In other words, we're aiming for a thousand words in an hour. (Not every hour!!!)
To be honest we rarely manage this, but the system still works well. We set a time, turn email off (and in my case I plug in headphones and turn classical music on) and then we try to write without stopping for an hour.
In short bursts it's doable. I often get 500 or 600 words down which is much faster than my normal snail's pace. Of course, we still need to go back over our work to edit, reifne, polish and, sadly, sometimes delete... but for people who have to write day after day, these short bursts can be like a shot in the arm.