Set aside a good bit of time to write your first draft. List out the ideas that you want to include; refer back to the questions on the HSAC application and introductory worksheet.
Plan on at least 5 drafts and share them with people who know you well. Your committee, friends and family, can all give you valuable feedback. The folks at the writing center and career services are also happy to help with this type of statement.
This may be the one element of your application that distinguishes you from thousands of other applicants. Grades and exam scores don't really help you stand out; they just put you in a group of other students with similar grades and scores.
This statement often precedes any letters of recommendation. You make the first cut on your own.
General statements that could be written by any applicant should be rewritten and personalized.
Guard against being redundant.
Speak from example. Tell a story about an experience that helped you to know that you wanted to pursue this career. One or two examples help make your statement interesting and memorable.
Be active in your writing. Be direct, your audience won't bother looking you up to clarify points.
Don't waste words listing information that is somewhere else in your application.
Don't waste words telling them what a professional in their program is.
Don't do the job of your letters of reference writers; let us brag about you while you just tell them who you are and why you want to enter their program.
Tell your story in chronological order, it's easier to write and easier to follow.
Consider 3 or 4 paragraphs.