Been through two lots of personal statement nonsense now. This is how we approach it. Tell them not to think about writing it all at once. Get them to break it into paragraphs and it is much more manageable. Look on studentroom website for example personal statements but don't copy as they will find out!
Paragraph 1 - introduction / why they are interested in subject.
Paragraph 2 & 3 & 4 - demonstrate what they have read or done to extend knowledge of subject - for example work experience, further reading, trips, lectures, building something. If they don't have anything to write about then plan the summer holiday around some trips and lectures.
Paragraph 5 - extracurricular (trying to link them to the subject)
Paragraph 6 - Conclusion
Don't leave a blank line between paragraphs as every space counts and get lots of people to check for errors. They hate statements such as.....'I am passionate about Physics because....' At one Open Day a member of staff told me that they play personal statement bingo for obvious openings / gaffs and that they can spot which schools some of them come from, so he advised making it individual and written by the student!