Writing a draft is one of the later stages of the overall project, not the beginning. With a complex writing task, take the project management approach.
Mull over your writing project while you’re still doing the preliminary work. Don't wait until you actually begin drafting a 200-page proposal or 15-page white paper. Fifty percent of any writing effort is thinking. While you’re performing tests or visiting clients, you can begin to analyze your audience and decide how you want to approach the writing. Your subconscious will do wonders while you are going about your other activities.
Set up a task list with deadlines. Work backward from your due date. Depending on the length of the document, of course, you should plan on several days (several weeks would be better) between completion of the first draft and the final document. The interim cooling-off period between first and final drafts allows you to approach the revision stage objectively with a fresh eye. After you've counted backward from the final due date to the first-draft deadline, continue to count days until you have set interim deadlines for all phases of your work. Include deadlines for the following:
Calls to all people supplying information
Completed outline
Outline-review meeting with any other writers involved and whoever has final approval on the document
Requests for artwork to be prepared by others
First drafts from all writers of sections
Compilation of a complete first draft
Comments on the first draft from all reviewers
Artwork review meeting
Revision of the document
Final manuscript preparation
Final proofing and submission
Keep notes. Don't ever trust your memory. As ideas come to you about points to include, facts to state, arguments to present, or artwork to use, jot them down and toss them into a file. When you get to the drafting stage, much of your outline work will come from these bits and pieces.
Plan each section of the document. If you don't have time to organize your thoughts, how will you ever have time to revise the draft once it's down on paper? And that's exactly what you'll have to do if you string your information together without forethought.
Set aside blocks of time to write. You lose time and continuity in "getting into" a project each time you temporarily put it aside. If you can’t devote full days to the writing effort, at least faithfully devote two to three uninterrupted hours each day.
Psych yourself out of procrastination. Break the writing project into small sections and then do the easy parts first--maybe write the introduction or the section on procedures or compile a list of charts and graphs to include in the document. As your pile of "finished" pages grows, you'll gradually pass the halfway mark and become motivated to finish the more difficult parts of the writing task.
Inspiration comes only to prophets. The rest of us have to tackle writing just as we do any other project--with planning, determination, and coordinated effort.