I suspect that perfectionism is like alcoholism: you're always recovering, never recovered. It's a process you keep having to commit to. For today, I will show up at the page (Julia Cameron). For today, I'll write part of a shitty first draft (Anne Lamott). For today, I will not beat myself up for not writing more/better/sooner.
The following advice is mine only in the sense that it was given to me. I'm passing on, and elaborating on, what RY said to me a few weeks ago (under #4, here).
First, identify the origin of the problem. In my case, though there were no doubt a native tendency and some earlier influences, the pre-eminent cause was a magisterial figure in my doctoral program. He built the program; he taught generations of students; he hired or influenced the hiring of most of the other faculty who taught in the program; he was greatly respected by other scholars. He insisted on meticulous scholarship, based on original sources—manuscript sources, because you can't trust other people's editions. He had a photographic memory both for manuscripts and for secondary references. He never said, "Somebody wrote about that, let's see, about a decade ago, in either Journal X or Journal Y." No matter the topic, he'd reel off author, title, journal, year, and page numbers, without looking anything up.
He published important articles; and his wife once told me that when something of his appeared in print, he'd wander the house with it in hand, reading bits aloud and chortling. She believed that one should celebrate publications and enjoy them, not use them as occasions to find fault with one's own work, wishing one had done things differently.
But, as RY said, where is the row of books with his name on them? He was always working on a magnum opus, but it never came to completion. And come to that, where are the rows of books from his students? One of his most noted students publishes articles at a rapid clip: this scholar's way around perfectionism is to do lots of very short pieces, sent out in very rough form, then polished and expanded with advice from editors and readers. Several of the magister's students have produced multiple editions and essay collections. But it is notable that the most successful products of this program are those who are least associated with it: those who moved past their early training, often those who hated graduate school and wanted to get through so they could, figuratively, kick all their teachers in the teeth.
So, says RY, notice where perfectionism got this famous scholar. Notice that there is no point in trying to please him now.
Second, find someone productive upon whom to model yourself. (Throw off the cold dead hand of the magister.) Look at what this person publishes, how often, and under what conditions. Here I'm going to extrapolate a bit: if you know this scholar personally, ask about writing practices. What do his/her drafts look like when they go out? What does s/he expect editors and readers to contribute? (I am, however, reminded of a dinner-table conversation at Kalamazoo in which senior British scholars were bemused by the fascination of younger Americans with writing process: when [first thing in the morning or late at night?], where [home, office, coffee shop?], how [multiple screens open at once, papers spread across desk, long-hand, direct to computer, notes on photocopies or separate documents?]. To them, writing was just something you did, not something you talked about. Choose your audience.) Perhaps a student or friend of this scholar might have advice, if you don't know her/him.
Third, strive for clarity. Don't over-complicate your argument or your prose because you think they're too simple. Write your main point on an index card and post it over your desk. Stay on track. If other ideas bounce up, write them down for later pursuit, and return to your main argument.
Fourth, you need people with whom to share your work: to bounce ideas off of, to make suggestions for improvement, to edit, to tell you when something is ready to go. And not just that, but to suggest where you might send a piece. The social aspect of writing is hard for many of us, I think. Sometimes writing groups get set up at one's home institution, under official auspices; sometimes junior faculty band together. Sometimes regional groups bound by topic, period, or some other similarity (like women scholars) get together and read each other's work. Sometimes former students from a cohesive graduate program form a writing circle, exchanging work electronically. But if you've become isolated, then you'll have to find a way to reach out to someone. You may just have to ask. A couple of times recently, I've offered to read someone else's work, intending this to become an exchange; but they've demurred. Next time, I think I'll just say I have something I need a reader for, would appreciate feedback, and will of course reciprocate.
Fifth, finally, and according to RY, very importantly: do not expect too much of yourself. Do not plan to do more than you can do in a given length of time. You have to set manageable goals and then meet them, so that you can say, "Look, I am accomplishing things," instead of "Oh, no, I'm failing." Success begets success. Accomplishment leads to more accomplishment. Feeling good about meeting goals means you can feel good about writing, and wander around your house with journal in one hand, Scotch in the other, delighting in seeing your name in print.
How easy to give advice, and how hard to follow it! But I'm saying these things in an effort to make myself accountable for practicing what I preach.