Look at the personal autobiographical statement instructions if you have been given some. Schools or employers typically provide a word limit or page-length limit. Before you begin, note this limit. If you exceed this simple instruction, your statement likely won't be considered with as much weight, if at all. Note, too, any specific areas that the instructions tell you to cover---then cover them.
Consider your audience and write accordingly. For example, if you are writing a personal statement as part of a graduate school application, you'll probably want to focus on your academic achievements and educational history. If you are writing a statement for a potential employer to see, your focus will be more work oriented and more professional, with a focus on skills, experience and work-related education.
Consider centering your statement around a personal story. After all, when a review committee or potential clients want to find out about your accomplishments, they'll turn to your resume. Your personal autobiographical statement should be less of a resume and more of an introduction to the real you. A personal story typifying you and and your strengths does this. A story makes your statement more memorable too---which should be a key goal here.
Run a spell-check. If there are spelling errors in your statement, it will likely be viewed far less seriously than it otherwise would be. Read through the statement several times, looking for typos and grammatical errors. Ask someone you trust to read through your statement too; it's surprising how others often immediately see errors that you missed over several read-throughs of your own.
Allow a few weeks to pass before you turn in or post your personal autobiographical statement (if you have the time). In that period, you may notice something you missed or think of something to add or change.