I am happy to write strong reference letters for students I know well. The strongest reference letters are written by professors who have first-hand knowledge about a students' motivations, strengths, and skills. Strong letters give admissions committees information they cannot learn from looking at transcripts and application essays: information about the student's work ethic, problem-solving ability, inter-personal skills, and other attributes. These letters are fairly easy for me to write if I've worked directly with a student on a research project. They become more difficult when I haven't interacted with a student very much.
If you are not working directly with me but are working with one of the graduate students in the lab and think you would like a reference letter from me in the future, please make a point to stop by my office occasionally and tell me what you're up to. That will make it much easier for me to write you a strong letter.
If you haven't done research with me but have taken a class with me, I can still write you a reference letter. I am sympathetic to the fact that it can be difficult for students to find multiple professors who can write them strong letters. But if you haven't ever come to my office hours or otherwise allowed me to get to know you, please be aware that I will not be able to write you the kind of detailed letter you need to get accepted into graduate or professional school. If you have two other strong letters, you will probably be fine. But if you don't have any professors who know you well enough to write a strong and detailed letter for you, then you might want to wait a year before submitting your application and get to know some professors.
If you want me to write a reference letter for you, please send me an email with the following information:
Your name and a reminder of how we know each other
Where you are applying, the deadlines for reference letters, and information about how to submit the letters
Your resume and unofficial transcripts
A draft of the application essay you are submitting and/or an explanation of why you want to go to graduate or professional school and your career goals
If you've done research in our lab, write a brief description of the work you did and the outcome.
Any other information you think is relevant and will help me write a stronger letter. This is not the time to be shy or humble - I would be happy to brag about your accomplishments on your behalf, so let me know if there's something you want me to tell the admissions committee.
If you ask me to write a letter for you and don't hear back from me within a week, please send me another email. Also, once I have agreed to write the letter, a gentle reminder or two as the deadline approaches is always a good idea.