I want to focus not on the formulaic or "common sense" aspects like having a cohesive message or good grammar, but on what will address the needs of the reader -- the person who will actually evaluate your essay. Remember that the idea is not to just regurgitate information that's readily available or common to any applicant. The idea is for the reader to know something unique about you and how you've approached your situation, what's unique about that situation, and what drives you. What obstacles have you encountered, how you've responded, and what you hope to accomplish. The major idea of a good supplemental essay is what makes you tick, as they say. The good news is that you're likely the best expert on you. So take advantage of that. When interviewing anyone for a position or award or anything at all, what I've always found most useful is getting to know the personality of the applicant. The original application or resume or whatever initial documentation the person has provided should have all the objective information. I've generally been after subjective information that only that person could possibly provide. Sound good?
It's totally natural to be a little stressed about these essays - this is a lot of pressure. Don't overthink them!
Take a step back and think about your life story. What struggles or unique paths did you encounter growing up that make you the student and person you are today? What is your plan for college? Do these two intertwine? Something that will stand out in the admission process will be a very personal and unique essay that tells the reader who you are and why you want to go to their school.
For example, a friend of mine used the story of her mother's experience with breast cancer and their journey. This lead to her passion in becoming a nurse and thus applied to a school in Idaho known for nursing/medicine. Her story fueled her passion, which in turn showed how serious she planned to take her studies. These elements are very inspiring and impressive to the readers in admissions offices.
Good luck in your collegiate goals! Take a deep breath and think about why you're applying to these schools and what kind of student you plan to be in their communities. You got this!