I teach an upper-level microbiology lab in the fall semester. It meets twice a week for almost three hours each session, which means I spend a good bit of time with my students in a more personal environment than a large lecture hall. Because of this, several students over the years have asked me to write letters of recommendation for them for graduate schools, internships, medical school, jobs, and, in one case, the Navy.
From the institutions who will receive the letters:
1) Academic achievement is important, but so is character and interpersonal skills. In addition to affirming the applicant's knowledge and practical skills, the institutions wanted to know if the applicant was teachable, respectful, and worked well with others. I also included if a student was punctual and responsible (turned in assignments on time and kept his or her lab area clean, and so on).
From the students:
2) Always ask permission before putting someone down as a reference. Never give out a private phone number. I once received a request from a program for information on a student who never spoke to me about being her reference. She also gave this program my personal cell phone number. I never gave that to her, much less gave her permission to give it out. I didn't get her a bad reference, but neither was I effusive in her praise, which the representative I talked with probably picked up on.
3) Request to meet the one doing your recommendation letter in person. If it is possible to meet with the instructor you want a recommendation from, contact him or her by email or phone to ask about meeting. Tell her you want to discuss providing a recommendation and offer to bring any additional information that might be helpful (such as a resume). A few students did this, and I found it a mark of professionalism as well as helpful in writing a good recommendation letter. I could easily have said no and requested that they send me any information I needed, if I didn't have the time. But it was nice to see them again and ask them in person what their goals were and why they wanted to pursue their particular path, as well as what qualified them for it. In addition to refreshing my memory of who they were (it's easy to forgot which face goes with which name sometimes), it showed courage and good interpersonal skills in them. It also gave me a stronger sense of what I should say in the recommendation letter.
4) Send, or offer to send, a resume and other information that might be helpful. I don't know everything about my students' dreams and goals and qualifications. A simple resume as well as a brief statement (in person or via email if we can't meet) of why the student wants to be in a particular program or go into a particular field of study is helpful. Also, the student would be wise to remind the instructor as to when and how they know one another (give year and class in initial email request).
5) Be clear in expectations (and for me personally, be diligent) . One student, whom I particularly liked (fortunately for me), asked me to do recommendations for summer internships. She told me what to expect regarding the type of recommendations (letters verses forms, online verses printed). I agreed and was expecting more than one request to come in by email for her. About eleven or twelve came in! I was not expecting that many. While I think mentioning exactly how many to expect would have been good, I admired her diligence in searching for and applying for so many opportunities. It was an encouragement to me to keep trying, because I'm all to apt to give up after a few job applications or agent submissions.