Do:
Make your statement specific to the subject you want to take, not a general CV.
Check what admissions tutors look for - use books/websites (we've got a guide to the best books/websites on another page).
Be passionate about your subject.
Instead of just describing your work experience, reflect on it and discuess what you learned.
Talk about how you would cope with a heavy workload.
Show how your hobbies are relevant to your subject - or use them to show how well rounded you are!
If you're taking a gap year, talk about your plans.
Aim for a strong opening and ending.
Take care over presentation and spelling. You're generally advised to write in paragraphs.
Try to find out what your reference will say; your statement should complement rather than duplicate it.
Look at the information written in the university prospectus' and write how you can cope with the key factors of the course.
Don't
Copy someone else's statement - UCAS has software that can spot plagiarism.
Try any gimmicks - somebody will have done it before you!
Duplicate anything that's already in your application.
Lie!
For more help use these books and websites:
Studential
How to Complete Your UCAS Application 2012 Entry: The Insider Guide to Making a Winning UCAS Application
The UCAS Guide to Getting into University and College
Heap 2012: University Degree Course Offers: The Essential Guide to Winning Your Place at University
Personal Statements: How to Write a UCAS Personal Statement (])