When planning your master's thesis, decide not only what you want your topic to be, but also what purpose your thesis will serve. Think carefully about what the finished thesis will prove and how it will be relevant to potential readers, ensuring that your time isn't wasted crafting a purposeless thesis.
Don't attempt to take on the task of writing a complete thesis all at one time, but instead break up the task into more manageable parts. Plan to write at least one page a day, gradually building your thesis to the appropriate size instead of chaining yourself to the keyboard and pounding out the entire document in one sitting.
Study the thesis format prescribed by your school before you start to ensure that you follow it. Failure to follow this format can result in getting marked down, so sticking to it is quite important.
As you write, keep your audience in mind. Think about what pre-existing knowledge your audience brings to the table when deciding how much detail you need to give in your thesis. If your topic is highly specialized, it may be unlikely that your audience has any background in it at all, making it more important that you give some basic information at the start to ensure that the readers can understand the more complex theories and ideas you present.
Your thesis should be long, but it shouldn't contain pointless ramblings. Get to the point, keeping your writing as concise as possible. If you don't, you will likely lose readers who are not willing to wade through extraneous words to determine what you are trying to say.
Your first draft of your thesis is far from a finished product. Read though this initial copy carefully, seeking ways to improve upon it by moving the parts of your paper around until you have craft the perfect arrangement. After your major revisions, turn your attention to smaller edits, perfecting your thesis and readying it for submission.