Sometimes the hardest part about writing a memo, report, or proposal is getting the words on paper. The key is to write all of your thoughts down before you can overthink them. Then decide how to best structure them. Once you've done that, rapidly write your first draft. Don't wait for inspiration. Write against the clock, giving yourself 5-10 minutes for each section. Resist the urge to perfect as you write. Save the editing until the draft is finished. If you find yourself stumped, move on to a section you're more comfortable with and come back to the problem area once you've found your flow.
Source: Today's Management Tip was adapted from the HBR Guide to Better Business Writing.