Research the graduate program. Know who the faculty members are and what they specialize in. Know what classes the university offers in your area of interest. The shape of your thesis may be determined by the specializations of the faculty members and the classes you are able to take. Therefore, do the necessary research to make sure that the program of study fits the topic of your proposal.
Propose a project you care about. This may seem like a no-brainer, but many people are uncertain about what they want to study, or they simply propose something they think will appeal to the application committee. If your preferred thesis topic does not fit the program, find a program that better fits you. You may spend a year or more working on your thesis, so it is important that you be passionate about the topic.
Show the committee any background knowledge you already have in your chosen subject. List classes and independent studies you have completed, professors you have worked with and papers you have written. Graduate schools are most likely to admit well-prepared students who have a demonstrated interest in their program of study.
Outline your preliminary research question(s), resources you might use, classes you plan to take and faculty members you would like to work with. You will have to do some research on your subject to find useful books and articles, but the committee will be impressed with your well-thought-out proposal.
Tell the committee why you have chosen this subject. Explaining how you developed interest in this topic demonstrates your serious intentions as a scholar and makes your application more personal for a committee who may only see you on paper.
Make the proposal as detailed as possible within the parameters of the application requirements. Be very specific about what you wish to study and why and how. The more detailed your proposal, the more purposeful and serious about graduate school you will appear.
Remember this is only a preliminary proposal and you will be able to change things later on. Writing a detailed proposal will not lock you into this thesis. But to get into graduate school, you must first demonstrate your intent to research a specific question or topic. Take the preliminary proposal seriously and you will be in a good position to change it once you have started attending classes.