Needless to say, the statement should be well written using formal language - contractions and colloquialisms are a no-go. Aim to keep your language varied (avoid repetition) and keep sentences under 25 words. And avoid the passive voice! Your goal is to sound clear and intelligent which is most evidently reflected in the way you discuss your motivations and merits.
First and foremost, you should state your reasons for applying. This means justifying the choice of programme as well as the choice of university. Ask yourself: what is it about the course you find intellectually appealing? Is there anything about it that is unique or especially aligns with your interests? Why have you chosen that institution in particular? Is it for its academic reputation, or the location too?
It is critical that you are specific in these areas in order to demonstrate a genuine and tailored interest.
Second, you should talk about your bachelor's degree and how it has prepared you for further study. If your bachelor’s and master’s subjects are closely related, this should be relatively straightforward. You will be able to go into detail about specific modules and topic areas that engaged you most and you want to build on in postgraduate education.
If, however, your undergraduate subject is somewhat different to your master’s programme, you need to be a little bit clever by finding knowledge and competencies picked up in your prior subject which can be extrapolated to your new subject. For example, if you studied chemistry and want to do a postgrad in international affairs, you could talk about how you have learnt how to use cold logic and data analysis to form reasoned conclusions which you now wish to apply to the global landscape.
Third, it is necessary to mention the skills that you can bring to the degree. Whilst these can be things you learned in your bachelor's, such as numeracy skills or programming, they can also be things you have picked up in your own time - or better still, in a professional capacity. For instance, if you are interested in journalism, it would no doubt be worth setting up a blog of sorts to demonstrate your written investigative skills. Or, if you are applying to do an MBA, get some work experience at a local firm. These skills are accessible, the opportunities are there, but you must be proactive and seek them out for yourself. You can use these experiences as concrete evidence of your abilities.
Goals. This is arguably the most important part of the application. While obtaining a bachelor's degree is becoming increasingly common in the UK, and is made fairly accessible through the student loan scheme, having a master’s degree is not necessary for most lines of work. Thus, you need to give good reasons for why you want to do one, and the strongest way of doing so is by outlining your career goals.
There are different ways of doing this depending on where you wish to take your master’s degree. If you are doing a master’s because you want to work in academia, then explain this to admissions. Tell them about your specific research interests and how your choice of master’s will springboard you onto a PhD.
If you are doing a master’s to improve your employability, then you should discuss what line of work you want to go into afterwards and how your master’s will help you get there. If you are doing a practical master’s such as journalism or an MBA, this will be quite straightforward. If you are doing a history master's, however, and want to subsequently work in law, you will have to draw a less direct link. For instance, how the close study and evaluation of historical texts will give you the information analysis skills needed to go into a vocation which demands the comparison of the strength of facts and arguments.
Make sure to get a friend to proofread it, or better yet a staff member to give you some feedback once you're done. Remember, the personal statement is only one part of the application, so don't let it take over your life and wear you out. Good luck!