Refine your topic. Remember that your focus is on presenting an idea for a course of study. Do not get sidetracked by trying to work on your actual thesis at this time; instead concentrate on writing a clearly worded and easy-to-assimilate statement of your topic. Remember there are very clear writing guidelines for forming a proposal to make it easier for you to complete the task at hand, which is getting your proposed coursework approved by the department to which you are applying.
Summarize the available literature covering your area of study. Take short notes, and focus on a synopsis of the materials which you are reading in preparation. It helps to concentrate on the most recent materials to give a more relevant feel to your proposal. The proposals that will win approval will be those that show great promise of contributing to the further scholarship of your topic.
Start your writing by creating an outline for your proposal, carefully including each major point you plan to make during your defense. Remember that consistency is key. Although this may seem a bit dry and technical, it will make it easy for you to assemble your final draft of your proposal without loosing focus.
Prepare for changes. In the course of your scholarship, you will change your mind about certain areas of your proposal and those will more than likely be reflected in your proposal. This change indicates that you are capable of growing as a student and will be anticipated by your committee at the end of your studies.