A few last thoughts on writing letters of recommendation.
I keep a set of templates in my letters folder. I've got essentially three levels for students:
1) Students I don't know you but you took a class with me. The class had >40 students (often 150+). I usually ask for a CV. If I and the student have time, we meet, so at least I can remember the face, if they spoke in class, or anything else memorable. This letter tends to be one paragraph. It mentions the rank or grade of the student, and usually 1-2 facts that are specific to the student.
2) Students who took more than one class with me. You came to talk to me. I've got more than 1-2 facts I can say about you. These are usually two paragraphs. One with the facts about the class or classes and the objective facts about the grades the student got, and one with specifics that give more info on the student.
3) Students who worked with me. These are usually two-three paragraph jobbers. The first paragraph still tends to have the objective facts (Mark worked 10 hours a week for two terms, a total of 36 weeks, and received credit/or was paid). If there are two sets of interesting specifics (ie class vs work) then they are in two paragraphs.
General points that make writing easier for the more complex or lengthy letters:
I almost always ask the person for whom I am writing to give me a list of what they've done in my lab. I tell them to include any specific incidents they remember as being important. I don't ever include anything I don't explicitly remember, but often they jog my memory about something that is good for a letter that I've forgotten. I find their bullet points are very useful for me, yet in list form easy enough for me to read.
I try and tell the student what kind of letter I'm writing. So if its going to be 1-2 paragraphs, I say that I don't know them that well, and that this is all I can say about them. Sometimes they go ask someone else, sometimes they say that's fine. But I do not worry if I can't say more. In the beginning, I used to ask the students to come see me, and interviewed them, got more information took notes, and crafted tremendous letters. That strategy didn't last for very long. It just took too much time, and I found that I was not giving the most help to the students who I thought were the best, who may have deserved the help. When I taught large pre-med, undergraduate classes, I would be asked for 5-20 letters in a short period of time. I also don't worry tremendously about making each letter a work of literary art. That's not a good use of my time.
I also try to be honest about the quality of the letter to the student. "I'm happy to write this letter for you, Susie, but you only got a C in my class, and we really never spoke during the term." I don't mind showing students the letter, but they don't get to decide if I send it or not based on what I say.
Writing letters is an important job, and important to the students. However, in the panoply of things I do, and the importance to my career, its not so great. Balance, balance, balance.