All PSs will have an introduction in some form. This needs to start in an interesting way, to draw the reader in straight away. Remember that admissions tutors will read hundreds, if not thousands of them! 'I am applying to study BSc Psychology' for example, is (a) a waste of characters, as the admissions tutors will be from the psychology department and (b) a very boring way to start a PS. Avoid cliches such as 'I have always been interested in' - technically that can't be true, as it would have not been the case as a baby! Also, it is best advised not to use quotes in your PS - it is meant to be personal to you, so the admissions tutors want to know what YOU think, not what someone else does.
Use the introduction to possibly talk about HOW you got interested in psychology (although don't say 'studying AS Psychology has made me want to study this subject further, ' as it will bring up the question 'well why did you choose to study AS?'). Also, don't make it too vague, by saying things like 'I am interested in how people interact' or the cliched 'I am interested in finding out more about myself'. A psychology degree is an academic qualification, and this is not what you will cover. Instead, mention an area or two and say WHY it/they interest you, albeit briefly. You can go into more detail in the subsequent paragraphs.
Academic content should take up approximately 2/3 of your PS. It can be split into two: college academics (A Levels etc) and academic interests/activities outside of your formal education. The latter is obviously more interesting, as it shows more motivation to know more about the subject you are wanting to spend 3+ years (if you include a relevant postgraduate course) studying. However, you may not want to separate them that crudely - for example, covering something at A Level may have enthused you to discover more about that subject, so put it together.
This is not the place to list your A Levels and what you've done in them. It is also not the place to try and link everything to psychology, no matter how tenuous the link. Try and avoid saying 'Studying English literature has improved my essay writing skills and helped me construct concise arguments'/'Mathematics has helped with my data analysis skills'. These will be pretty self-evident and a waste of characters. Instead, talk about what in your A Levels (related to psychology) has interested you and why. If you found it particularly interesting, you may want to briefly talk about any experiments/studies that you did during A Level Psychology. Don't just explain what different areas of psychology/studies etc are about - reflect on them, say why they were interesting. Admissions tutors will be familiar with what (e.g.) cognitive psychology is about!
The second part would be far more interesting. This can come in a variety of forms: reading undergraduate level text books/reading academic journals (including those aimed at college students)/work experience. The last one is far more difficult to obtain, with confidentiality surrounding a lot of what psychologists do. You may get to talk to one about their job, however. Other relevant work experience could include volunteering in a mental hospital or in a school (development, learning, memory, learning disabilities etc); you don't need to mention the name of the school, just say 'a local primary school' (e.g.). When talking about these things, don't simply name drop - anyone can do that. Instead, say WHY you thought it was interesting and if you can, add some sort of evaluative comment (e.g. about strengths/weaknesses of the study, how useful the findings are).
A simple example of how to say 'why something interested me': 'Milgram's famous study of obedience to authority interested me, due to the shocking and unexpected results obtained, even from well educated participants.'
As for what things to read, try and avoid the 'pop psychology' books, such as 'The Man Who Mistook His Wife for a Hat', or really famous studies, such as Piaget, Zimbardo etc - I have read so many PSs that have included that, and it doesn't really include much actual psychological content. Look for what kinds of modules you'll be studying - e.g. there isn't much point in saying you're interested in psychodynamic theories (dreams, unconscious thoughts etc) if your chosen universities do very little (or none) on the subject. Some journals that are aimed at college students include: The Psychologist, Psychology Today and Psychology Review. Some other possible reads are mentioned in the 'Psychology at University - FAQ' thread, or you may want to consider using something from the 'Interesting and Useful Psychology Resources' thread or the 'Books to read for wider psychology reading, and for university preparation' thread.
Something else important to note - don't go mentioning Freud in your PS! Undergraduate psychology is far more scientific than A Level, so Freud is going to only get a brief mention, if any in the whole 3 years of the degree.
This section is for anything that is not specifically related to your interest in psychology. This part should be short, a maximum of 1/3 of your PS. It can include things from school/college as well as in your free time (including a part time job). For school/college, you may want to talk about peer mentoring, prefects. Remember to keep your sentences short and snappy. If they're long, people get bored and stop reading. Cut out all unnecessary words. Don't start your sentences with verbs unless absolutely necessary (e.g. “Being a prefect” is too informal). Say what you did/do, then what you learned from it, and sometimes explain why that is useful, but not at the expense of it being interesting. Don't repeat things you learned- you only need to demonstrate characteristics once each throughout the statement. You don’t need 3 examples of how you can handle responsibility! Other characteristics you can talk about are team work, communications skills, leadership, confidence, etc. Don’t worry if you don’t include them all. If it is just going to sound fake and boring, it’s probably better not to bother. You do not need to relate everything to psychology - you are allowed to have a break from it, even at university!
As for your interests outside of roles of responsibility, keep it very brief. Sport and musical interests are generally good ones to include and just briefly say why you enjoy it. Less important are things like 'I enjoy going down the pub with my friends/shopping/going to the cinema' etc. As long as you have SOMETHING written about your extra-curricular activities (if just to show you exist outside of college), it doesn't matter how many. Quality is better than quantity, and you want this section to be brief, so there is no point in listing a load of activities. Think about how they've helped you.
This would also be the best place for international students to briefly mention why they want to study in the UK.
If you are deferring entry, it would also be useful to include any gap year plans and say why you are doing that.