Many schools openly declare how they weigh your personal statement among other admissions materials. Your essay may be the single most important factor, or it may be simply a tie-breaker. If the weighting of application components is not obvious, try interpreting data about previously-admitted students. If the school is highly selective but the average GPA of its students is lower than you might expect, the admissions board probably greatly values the personal statement.
Great personal statements are written for their audience, so research the admissions staff of the college. For doctoral programs and smaller schools you may be able to learn the names of the exact individuals who will read your essay. If you cannot find names, conduct research or talk to current students to find out the priorities of the admissions office. If you know the reader greatly values something specific, like volunteer experience, showcase your applicable background in the essay.
The personal statement represents intangibles that may not show in your grades and test scores. Hardships you have overcome make a compelling story, but you do not need to have a truly extraordinary life story for a great essay. Tell a relevant story that explicitly demonstrates your committal to your studies or career goals. Consider how your background has led up to this next academic step being logical and necessary for your career. Your personal statement shows how you are especially prepared for and interested in the program.
Try to assess your own writing skills objectively. If you are a natural writer, let your talent shine. If spinning a narrative tale with wit and humor is not your strength, do not feel forced outside of your comfort zone. There is no reason to feel pressured to write in a certain style, and doing so may expose your weakness and obscure your other talents. Admissions boards can smell phoniness from a mile away and an honest essay is better than one that seems stretched with fluff.
When facing a specific essay question, do not simply edit another essay to make it conform. Spend time pondering the question and how you can address it directly. Consider the reasons the admissions board has for suggesting the prompt. Questions about your interest in a special program are likely intended to elicit a description of your passion and commitment. If the prompt calls for your thoughts on a hot topic, the admissions board may be simply looking for strong rhetorical skills.