Demarcate your outline between the summary section and the suggestions section. This will allow you to focus on one aspect of the proposal outline while your head is unclouded by your thoughts on the other aspect. This is important to ensure that you remain flexible should your dissertation committee suggest any significant changes to either your summary or your suggestions section.
Break down your summary section into appropriate headings. Since the summary section of your dissertation proposal will emphasize your understanding of the literature pertaining to your topic, from the topic's beginnings to present day, you could use subheadings such as "Origins," "Early History," "Late History" and "Present Day."
Expand each heading into significant and detailed subheadings. It is important to work from general ideas to specific ideas. This will keep your thoughts organized. For example, in a heading entitled "Origins" for a dissertation that promises to focus on rhetoric, you might include subheadings entitled "Plato," "Aristotle" and "Cicero." Though this might require you to include more information than you actually plan on using in your proposal, it is important to add as much detail as possible to your outline. It is easier to subtract information later than it is to add it.
Flesh out each subheading with bulleted lists of relevant researched information. Include citations with any quotes or attributable material. This will keep your notes organized and protect you from accidentally plagiarizing the words of someone else.
Gather feedback for your proposal outline from colleagues and even members of your dissertation committee.