What should a good personal statement include?

A great personal statement will show universites that you are so curious about the course you are applying for you that you want to study it independently for three years.
The ways to achieve this? Choose interesting topics from your relevant a levels to the degree or from the degree itself and read about these topics you find interesting from books or any external sources. Then write about any topics that you found particularly interesting in your personal statement. This shows universities that you are generally interested in your subject due to you extensive reading around the course you are applying for.
Also include in your personal statement any work experience/ extra curricular activites that gave you skills that you feel like will prepare you for university life.
If your personal statement is for an Oxbridge application however, the personal statements should only include interests and work experience that are related to your chosen undergraduate course, as Oxbridge are extremely unlikely to value any skills irrelevant to the course. Most Oxbridge personal statements are jam packed with topics students have found interesting revolved around their chosen degree course, which is more than sufficient for any other university personal statement application too.