It is essential to answer the prompt provided in the application instructions. Typical prompts will ask you why you want to attend this school, or will ask you to describe an important experience in your high school career, or elucidate your career objectives. You should make sure your statement answers the question clearly and directly--nothing alienates an admissions committee more than a personal statement that was written generically for several applications, or even worse, written for another school.
Outline why you are motivated to attend this school. Theoretically, you already know the answer to this question, because you have researched your educational options thoroughly, and are ready to articulate precisely why this school is the best for you.
Prove to the committee that you know what you are talking about when you demonstrate your motivation to attend. Refer to a powerful lecture you once heard by a faculty member, or describe what you have heard from other students about campus life (don't emphasize the party aspect of college). You should avoid speaking in bland generalities like "smart professors and a nice campus"; it's better to refer instead to, for example, the "reputation of the History Department and the atmosphere of the Main Library Reading Room."
Admissions Committees may read hundreds of applications at a time: Take pity on them by writing something unique. Your personal statement will stand out if you describe some powerful personal experience (especially if it motivated you to apply to this college). Avoid talking about reaching the semi-finals in the municipal debate club contest; talk instead about what you learned in your summer job working alongside a newly arrived Sudanese immigrant.
Most of all, it's important to be confident. You shouldn't apologize for your grades or your test scores, and certainly shouldn't brag about them, but write in a way that shows the committee that you feel that the college would benefit by having you on campus, just as much as you would benefit from being there. Tell them you're completely committed to this application because you really believe this is where you belong.