I've just sent off my ucas application for engineering and here are a few of the things I talked about, you don't neccessarily have to link everything to engineeering, some of it can be general skills for higher education
Why I became interested in engineering and why I'm passionate about it. (intro)
Academic, what parts of your subjects you enjoy and why, what skills you have gained (independant learning) and also any stuff that you have done specifically academically that others may have not. (First paragraph)
Any work experience you have done and any other enrichment activities you have done relating to either engineering or university. (2nd paragraph)
Other responsibilities/hobbies/jobs that you have gained skills from that may be revelent (3rd Para)
Why should the uni pick YOU and what would YOU gain from doing the course (Final Para/conclusion)
Hope that helps and gives you ideas on how you COULD do your PS. I can't tell you what exact things to put down because it does have to be specific to you, and what things you have done that mean the uni should choose you for the course. Think of it this way, everyone who is applying will have roughly the same grades, thats a given. However the personal statement is your only chance (kinda along with your referee's reference) to tell the uni why they should pick you, what makes you stand out compared to just the grades, it also is your only chance to show them about you as a person.
Hence there is no real definative way to write a perfect personal statement it all has to be personalised to you. Does you sixth-form or college have a personal tutor (or even a friendly subject tutor) that you could go and have a chat too about your personal statement.