As a proofreader, I have read many MSc dissertations over the last few years. When I'm proofreading an MSc or a PhD dissertation, or an academic paper, I always try to put myself in the position of the examiner or reviewer and I always work under the assumption that the reader is unfamiliar with the details of the work. The following are a list of style tips I think are useful, probably for all academic writing, but especially when writing MSc dissertations.
Always keep the reader (ie the examiner) in mind, and make it as easy as possible for them to read and understand your work. Remember that your goal is to demonstrate what you have done and what you have learned from your research. Assume your reader does not know your topic as well as you do. If in doubt about whether to include an explanation, you probably should include it.
Avoid long sentences.
Avoid the passive if possible: "the dog barked at the postman" is better than "the postman was barked at by the dog". There are only very few exceptions to this rule such as if it will make the sentence very long and complicated.
Avoid slang - it looks sloppy and unprofessional. Always try to find a more formal term.
Avoid long, uncommon words if possible. Try to explain things in short, simple terms.
Read your work through - and then read it through again. And if possible, ask a friend to read through it. If English is not your first language, or even if it is but you know English grammar and spelling is not your strongest point, then be realistic about whether you would benefit from using a proofreading service.
Stick to the facts. Do not try to persuade your readers with emotional arguments or your own personal opinion. Try to be objective and justify all your arguments with clearly stated facts.
Remember that not all proofreading services are the same. Take the time to find one that you are happy with. If possible find a proofreader with knowledge of your subject, since they will have a better idea of the vocabulary and phrases used.