Here's the list:
(1) Show it! Often we procrastinate because we’re afraid to show our work to anyone. (“Afraid” is probably putting it lightly—we’re often terrified.) So stop hoarding your work and start showing it. But be judicious: there’s no point in showing to clueless or callous people. Show only to kind supporters who “get” what you’re trying to do.
Start now! Show bits and pieces, or the whole thing. Invite any feedback, or certain kinds of feedback, or no feedback at all. (Tell your audience what you want!) The showing, not the feedback, is the important part.
(2) Finish small stuff. Finishing is a skill you can practice. If you’re a fiction writer, write anecdotes and vignettes. (Bring them to completion, and then show them.) If you write nonfiction, write up (and show) one small point instead of several big ones. If you’re stuck on a complex email, write (and send) several small ones instead. (Here’s how to overcome email overload.)
Then move on (gradually) to finishing bigger stuff.
(3) Reduce the scope of--a.k.a., shrink--your unfinished project. Make it as small and easy to complete as possible by jettisoning subplots, subthemes, auxiliary topics, and everything else you possibly can. Use one or two examples to illustrate each point instead of three or four. Be ruthless! If you do this right, it will probably hurt a little. (Or a lot!) But you’ll be glad you did it. If you’re afraid you’ve cut too much, ask your mentors. You probably haven’t—and, by the way, the jettisoned pieces can form the kernel of other projects. (Look how prolific you are!)
(4) Stop trying to force it. Assuming we’re not being perfectionist or there’s not an “environmental” problem like bad management or a lack of resources, the only reasons work stalls are: (1) we don’t know enough about what we’re trying to do, or (2) we’re trying to force it in a direction it doesn’t want to go. So, do some more research if you need to, but in any case, stop trying to control the work, and let it flow how it wants to flow.
This advice would seem to make the most sense for “creative” projects, but it’s amazing how well it works for even “routine” ones, like grant proposals and office reports. Try it! (Don’t forget to work nonlinearly.)
(5) Ask for help. Always ask for help. Ask early and often. Ask even if you don’t think you need it. Empowerment is the foundation of all productivity and asking for help is one of the most empowered, and empowering, things you can do.
Oh, and don't forget to...
(6) Give yourself frequent rewards while you're working! They help keep you motivated!
All of the above applies to all kinds of work, not just writing. Got a big analytical project? You can also show bits and pieces, finish small bits, reduce the scope, let it flow, ask for help, and give yourself frequent rewards.
Good luck with your project!