Write your abstract. The abstract is a brief synopsis of your entire thesis. This abstract will not be your final one, but it will help you organize your thoughts and let you see where the holes may be in your dissertation.
Make an outline of your dissertation. Lay the whole thing out on paper before you get started. Organize your ideas so that they flow the most logically. A dissertation is a lengthy body of work, generally over one hundred pages. Go into it organized.
Know the most recent developments in your field and study the latest writings. Mention work other than your own that relates to your dissertation work. Critique what you feel didn't work or is inaccurate in the writings of others, but don't criticize just to criticize. Offer objective arguments with evidence.
Be clear and straightforward in your writing. Lay out your problem, the possible ways of studying the problem and which methodology you chose. Explain your studies, your results and what impact your study has on your field.
Use figures and data. Hard figures and data that back up your study raise your dissertation above just your opinion. They show that you have gained the knowledge necessary to succeed in this high level of education and that you know how to apply other people's data to your own work.
Edit and revise. You've gotten all of your ideas out. Now is the time to polish them and make them as cohesive and well-written as possible.