Type whatever comes into your head, even if it seems like garbage. After you've got a lot of text out, turn the knob back up and edit what you've written into something sensible. Another mistake is to imagine that the whole thing can be written out in order. Usually you should start with the meat of the paper and write the introduction last, after you know what the paper really says. Another cause of writer's block is unrealistic expectations about how easy writing is.
Perfectionism can also lead to endless repolishing of a perfectly adequate paper.
This is a waste of time. Think of the paper you are writing as one statement in a conversation you are having with other people in the field. Writing letters is good practice. Most technical papers would be improved if the style was more like a letter to a friend. Both practices have other substantial benefits.