The best piece of advice I ever received was when you ask someone for a letter of recommendation, ask them whether they are willing to write you a good letter of recommendation. If they hesitate at all, ask someone else.
Make sure the person you are asking is also someone who can speak to who you are as a student/professional, your work ethic, and likelihood to succeed in the program you are applying to. Admissions offices don't care how impressive the person (ie. an expert in the field, a well-known figure in the industry you are applying to, etc) you have a LOR from is if they can't speak about you in a way that showcases they know you well and gives the admissions committee more insight about you.
As much as you can, provide your recommender with any resume or additional documents of extra-curricular, information you would like for them to mention, or anything you feel is important. Recommenders want to provide you the best letter they can, and this way you not only show initiative, but you make their job easier by providing them relevant information they can use in the letter.
I worked for four years in an admissions office and can't say how many bad letters of recommendation I have received. This includes letters like "I do not recommend this student", and "I do not know this person well". It's a sure fire way to get a deny as letters are very important. Take the time to make sure the people writing your letters know you well. Take the time to sit with them and speak to them further about your career goals and aspiration and provide them with any documents or information they can speak to. Letters that tell my team more about who you are, why you are pursuing the degree you are, what your aspirations are, your work ethic, etc. carry so much weight. Letters of Recommendation really help the admissions committee really get a sense of who you are as an applicant, and your ability to know who will be able to best speak about you.
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