One of the most important rules in writing and public speaking is to consider your audience. While everything you write should idealistically be for yourself, you should always try to remember who will be reading your essay and what your audience is looking for (without compromising your writing style of course). This tip is especially important when writing to acquire a scholarship.
Make sure you research the scholarship. What do they look for? Questbridge, for example, is specifically aimed to benefit students from low-income brackets. So, it wouldn't be wise to write about your lavish lifestyle. This idea may seem trite, but I assure you that it can make or break your essay. Questbridge wants to know how poverty (and often other hardships) has affected your personal and academic pursuits. You may choose to speak about how you refused to succumb to your circumstances, or how your grades tanked because of a family member's illness. Whether you choose to write about your courage, your overcoming of certain obstacles, or even your failure at times, don't let it turn into a sob story. These topics act as an excellent launching pad to write, but your essay shouldn't be engulfed in depression; keep it upbeat (or at least end it on a high note).
Additionally, peruse the profiles of people who have already received the scholarship. Some scholarships have a page on their website that features a list of scholars. They may also have short bios. Use these to see what the scholarship recipients may have written about. The Gates Millennium Scholarship Foundation, for example, touts its commitment to help minority students (primarily those with in high academic standing in spite of poor social conditions). If you're applying, take a look at the students who received the scholarship in prior years. It doesn't hurt. You'll become more informed about what the foundation may be looking for, and that knowledge will undoubtedly manifest itself in your essay.
Remember to keep the prompt in mind as well. It is devastating to read a beautifully written essay that has nothing to do with the prompt. Scholarship committees are interested in you and how you pursue your interests, but are most likely looking for specific things from prospective recipients. They give you a prompt for a reason. Don't stray from the topic. Keep a mental note of what you are trying to say, why you're trying to say it, and how you can best say it. This applies to any piece of writing.
You must simply use what your audience wants to know to propel you into crafting something beautiful that incorporates your unique writing style and experiences. It's not like the Common App essay in which you have free range. You should narrow in on certain things. It's analogous to writing a supplement for a certain school. You research the school, its accomplishments, its mission, etc. Only then can you write. Do the same for scholarships. Research, research, research! Then, tell your story through the lens of a student who exemplifies the type of student the scholarship foundation is trying to help while staying on task. If you follow this, you will see many more positive end results.