Personal statements are hard and take time, good personal statements take quite a few drafts to get right. The most common mistake is trying to use bombastic language to try to impress the reader, this never works. It is a personal statement, it has to sound like you. If you try to look more intelligent, using big, uncommon words I can promise you that it will have the opposite effect.
1st Rule of Personal statements: be honest.
Don't lie. Simple as that, if you feel that you have to embellish it slightly, try to keep it to a minimum.
2nd Rule: Give them detailed examples which show you enjoy the skills or experience you are claiming.
Don't just brag. "I am awesome and have excellent interpersonal skills and work well as a team" means nothing. In fact, the word "skills" never needs to appear in your personal statement, show by example. "As vice president for the Basket Cases Society I organised a fundraiser, liaising with the owners and managers of the venue and coordinating a team of volunteers..." is so much more powerful. Tell them what you did and any difficulties you experienced and overcame.
3rd Rule: Tell them what you have learnt from your experiences, don't just list any shadowing/placements you have had.
What did you observe and learn from your experiences in a hospital? How did the doctors liaise with the nurses? If you were lucky enough to observe any surgeries, what did you notice about what was happening? How did you benefit from the experience? You should ideally show the person reading the personal statement that you have not wasted your time sitting in the corner twiddling your thumbs.
4th Rule: It is a PERSONAL statement.
You need to be happy with it, it is not your friend's, parent's or teacher's personal statement, you do not need to follow their advice if you do not want to, the universities who do read them, want to know you, not your extended family, make it personal, talk about yourself, your interest (academic mostly!), and about what you want.
5th Rule: You do not need an introductory or a concluding paragraph.
If you have space fine, but the people reading these know you only have 4000 characters, and if you take up the last 300 by saying how much you will relish and appreciate studying at such a fine establishment in the UK as it has been your lifetime dream, you are simply repeating what everyone else could (and frequently does), write (and the universities know that all your universities read the same statement).
6th Rule: Structure
Personally, I would suggest that you start drafting your personal statement with the following paragraphs. Don't worry, as your personal statement develops and evolves this will most likely change, but it is a reasonable starting point.
1st - Your chosen subject of study and why it fascinates you.
2nd - Elaborate on any research you have done, what have you read? and more importantly, where did that reading take you? Did you start reading one article only to then spend the next three hours learning about a tiny thing which was mentioned on the third page as it captured your attention? Try to read as much as possible. There is a reason why you don't study a subject at Oxbridge, you read it.
3rd - Have you done any shadowing? What did you LEARN from it, what did you observe, how has it benefitted you? Why are you a better prospective candidate from having done it? Universities love students who are observant and can teach themselves (as it's what you will be doing).
4th/5th - Your society/sporting activities outside the classroom. But most importantly again, what did you do? What did you learn? Just saying you were President of the Photographic Society tells them nothing, how many members did you have? Did you have any activities? Outings? Events? Did you organise them? How did you delegate?
In general, your personal statement should follow the structure of most important, to least important
The cardinal rule is that you should never copy anything from the internet or other people's personal statements, personal statements are routinely run through anti-plagarism software by universities, if you copy and paste from other people's personal statements, then the university is well within their right to reject your application, so do not let other people copy yours either.
Oh, and please remember, "Rules are for the guidance of wise men, and the obedience of fools." It is your personal statement, and if you can talk about your passion for your interest in Maths, Chemistry or stamp collecting for 4000 characters feel free to do so, feel free to ignore any advice you are given, but don't dismiss it out of hand.
Don't panic, the UCAS personal statement is the least important part of the application, but, it is the part you have the most control over, remember that some universities do not even read them, but some do, and what you write in it can form the basis or beginning of your interview, if you have one, so, for the love of all you hold sacred, if you mention you are particularly interested in a topic, or say you have read something in your personal statement, make sure you re-read it, or re-research it before any interview.
If you have problems starting to write it, try listing all the things you think you might want to include in bullet points, tick them off once you have included them. Very few people will be able to sit down and write it instantly, so it might be a good idea to start building your list slowly, over a few weeks, adding points as they come to mind.
Good Luck.
