<p>Say, you did research with a professor. You finished the project, but you probably weren't exceptional in it (or you had to have delays due to classes). Under which circumstances should you NOT ask him/her for a recommendation?</p>
<p>If a professor has a neutral or worse-than-neutral impression of you, don’t ask for a letter. However, just because you weren’t exceptional with a project (especially if it’s due to having to balance classwork) doesn’t necessarily mean the professor won’t think positively of you.</p>
<p>I think no matter how well a prof/employer knows you when asking for recoms you must ask them if they can write you a STRONG letter not just a letter. A stanford prof who sat on ee and bioE adcom told me this and stressed out that the strength of the letter matters more than the recommender. </p>
<p>When asking for LORs, you must explain that you are applying to competitive schools where it could all come down to how outstanding your LORs are, and then ask if they will be able to write you a strong and outstanding letter based on your performance in research/courses. Do this with all the profs who know you well and then pick the 3 that were most willing to write strong letters.</p>
<p>First email or call the prof and ask to make an appt to discuss the prof writing an LOR. If they indicate they are too busy, this is their polite way of letting you know they cannot write a strong letter. </p>
<p>if they agree to a meeting, bring a well written (and spell checked!) CV or resume, a summary or statement of purpose of why you wish to go to grad school in your chosen field and a summary of how the prof knows you (what you did in their lab or in their class). This type of performance demonstrates purpose and organization on your part and makes it simple for the prof to write a LOR. </p>
<p>Trust me, many of my students who would have received ordinary LORs received strong LORs by providing me with this info in a polite and professional way.</p>