Most top universities look for people who not only do well academically, but who also have a life and other interests outside of studying. Tell them your interests and maybe why you've opted for the course you have applied for (presumably the same course at many unis?). Maybe include your plans for the future. (Perhaps you want to study maths (or whatever) because you'd like to go on to teach it to young people or whatever. Say if you enjoy to go to the cinema with friends or have an interest in film and media (especially if it could be relevant to your course).
If you have sport interests or play in any teams at school, include these. If you play instruments or if you enjoy drama or dance etc include that. Include your achievements and hobbies.
There is no 'ideal' statement and as someone already said, if you try and make it fit a perfect mould, the universities will most likely see this and perhaps not believe it. They look for original and different things. I'd also recommend making sure you write it well grammatically and read it through several times, so you don't make silly errors.
That said, every university that I applied to was willing to give me a place there... and I wouldn't say I am overly smart, so I must have written a very good personal statement :-)
I heard of one guy who was the best student you could find, he got top grades in everything and was therefore absolutely positive that Cambridge would offer him a place... but they didn't because he had no hobbies outside his studies!!!