I would add an exception to @RoboKaren's answer. Economics programs tend to place very little weight on statements of purpose. Econ is perhaps a bit different from other disciplines because the first year is all coursework.
In a statement of purpose for an econ program one should highlight relevant previous courses/skills (especially math/stats) and indicate a few areas of interest (e.g. "Macro labor" not a dissertation proposal). Minimize the fluff.
Several economists (including Susan Athey [Stanford] and Jeffrey Smith [Michigan]) have composed helpful guides on grad school admission. I'd bet others have done the same in other disciplines.
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The general point that I hoped to make with this post (which was implicit but which I will now make explicit) is that the importance of the personal statement (and the nature of the application in general) is particular to a given discipline.
Why don't economists care about personal statements? First, it is costless for applicants to overstate/misrepresent their interests, talents, background, enthusiasm, etc. Second, given the other materials available to the adcom (see below), even a truthful statement is more or less superfluous. Third, deemphasizing the personal statement reduces the advantage enjoyed by native English speakers in admissions.
What do economists care about, then? By far the most important component is faculty recommendation letters. It means something if a faculty member is willing to say that someone is "the best student they've had in eleventy years in the profession". Suppose a faculty member misstates the qualifications of the applicant and the applicant is admitted (and perhaps offered a stipend). If the applicant fails out, then (1) the adcom will be disinclined from believing the recommender the next time around and (2) there may in certain cases be some informal social/professional consequences (i.e. "Hey remember that time you recommended that one kid and we wasted a spot and 25k on him just so that he could drop out in April of the first year?").
I don't think that this is a perfect system (or that admissions are perfectable) but that's just the way it works and has worked in pretty much any econ department for quite some time. Depending on your view of economics, it either more-or-less works OR goes towards explaining why econ is so messed up(!).
Ph.D. Program in Detail
Students who complete the Ph.D. program should have a thorough understanding of the principles of economic theory and its applications in various fields, along with an ability to think systematically about, and apply quantitative methods to, economic problems. The program gives equal emphasis to these two goals with formal courses and examinations, seminars, workshops, papers, and the dissertation. Students typically spend most of their first two years in course work, at the end of which they take the general examinations. The next three years are used to prepare their dissertations, although both shorter and longer periods are not uncommon.
Throughout the program, there are formal provisions for students to engage in original research work. During the second year, each student must prepare a research paper to complete the requirement in econometrics. Moreover, in the first two years, term papers are required in some subjects. The Department also offers students the option to satisfy one of the minor field requirements through a program of supervised research. Students are also encouraged to take part in workshops in their fields of primary interest.
After passing the general examinations, students participate actively in weekly lunch meetings and seminars, which helps them with their work in their preferred field of research. They prepare a third-year paper, which is due on May 1st. They continue to work on their dissertations under faculty supervision in the fourth and fifth years.