If there is one universal rule for proposals it would have to be plan before you write and write to the plan.
Your proposal should have specific goals and should answer specific customer requirements according to explicit evaluation criteria (whether provided by the customer or assumed). Your outline, themes, and text should all correspond. In most proposals this will only be possible if you plan the proposal’s contents prior to writing.
For each item in your proposal outline, you should plan which customer requirements it will address, which evaluation criteria, which themes, points of emphasis, bid strategies, what projects/experience it will cite, and what graphics/illustrations will be included.
Some proposal specialists use a process of storyboarding to accomplish this. With storyboards, you build the high-level proposal outline and then complete a storyboard for each one. The storyboard contains headings for items such as those listed above. Authors complete the storyboards to provide the information that needs to be presented in a section and complete the low-level outline. The storyboard acts as a data collection tool when working with multiple authors.
Some proposal specialists use annotated outlines, with the proposal outline, RFP, and other items listed above cross-referenced. Annotated outlines can also be used as a data-collection tool.
Regardless of whether you use storyboards, annotated outlines, or some other methodology, you should document your proposal plan in such as way that you can validate it prior to writing. For validation, you may wish to conduct a formal proposal plan review, with subject matter experts and management involved. Once validated, the plan becomes a blue print. In some cases, the plan can be turned into a checklist. This has the advantage of providing guidance to authors and an objective baseline to use during later draft reviews. In any event you want the authors to know what is expected of them to pass the later draft reviews so that they can get it right the first time.