My favorite place for free material is ■■■■■■■■■■■■■. The owner is selling her services. But if you go to her blog page – www.■■■■■■■■■■■■■/blog – you will find tons of articles with specific advice (not vague tips) about writing your essay. Above all, she preaches making sure that your essay is in the form of a narrative/story, which is that works best when trying to show college admissions officers who you are and why you belong at their school.
Hope this is helpful.