This is the key to completing your dissertation or thesis. Try the following strategies:
Tape a copy of the proposal to your computer, asking yourself if you have expanded on all of the ideas advanced in the proposal.
Work "within" your proposal, adding key words and concepts and then expanding upon them. This can help you integrate your research into a structure as you proceed while reminding you that you are making progress.
Write your first chapter early to make sure that subsequent chapters fulfill promises made in the introduction. This strategy helps some writers to focus their writing; for others, however, writing the first chapter proves difficult. In either case, writing the first chapter will remind you what your dissertation is about. See what works for you.
Use your thesis to focus your work. Let it dictate what you include; refrain from writing everything you know.