I currently have letters of recommendation on the brain, and while I am no means an expert, I have written hundreds over the past few years. I have also read many, from colleagues, samples online, and even ones written on my behalf. So, naturally, I have an opinion. Here are some do's and don't, just in case you're ever asked to do one (or 56, which is how many I'm working on this summer, so that I'm not buried this fall):
... create a questionnaire for the applicants, asking them things that you may want to incorporate in your letter. I ask for basic information such as GPA, extracurricular activities, and future goals. But I also get personal- what are hardships that the student has gone through? Do they possess any unique talents? What are their strengths and weaknesses? What are they proud of?
... make sure they know they will not get to read the letter.
... find out ahead of time what needs to be done with the letter. If it's being sent snail-mail, require the student provide you with an addressed, stamped envelope.
... make sure to change the information in your letter to fit the cause; if it's a scholarship make sure what you have written is applicable. I generally write one letter for each student and then adjust for different universities, scholarships, summer programs, or jobs. Bottom line? You don't want to write a letter addressed to Stanford when the student is using your letter for the Dell Scholarship.
... put some serious time into writing your letter. It really is something one should be flattered by (usually); a student respects your relationship enough, and trusts you, to write them something that may help them create future opportunities. The letter reflects them, but it also reflects you.
... try to add some personal, specific anecdotes in your letter to show a real connection to the subject. I try to reference presentations, specific essays, trends in classroom participation, etc...
... just regurgitate basic information about GPA, courses taken, and clubs involved in. All of that basic information is usually incorporated in the applicant's portion.
... forget to proofread. Who's going to take you seriously if you can't spell?
... write more than a page. People in admissions offices review tons and tons of applications and don't need to read a novel about how Susie Smith shows ponies on the weekends and then rushes off to moderate BINGO at the senior citizen's center.
... just use a form letter for everyone you write letters for. What if you have three kids applying for the same scholarship and the same reader reviews all three letters that are basically the same? Plus it shows a total lack of consideration about the individual.
... let students (or whomever you're writing for) take advantage of you. My general rule is that they must allow ten business days from the time of request for me to have the letter completed. If not, you'll get one day notice, and it will inevitably be the day grades are due, your kid has a doctor's appointment, and you're fending off a sore throat. I obviously don't generally turn kids down, but I do let them know that I can't be expected to write a high-quality letter in a short amount of time.
... try to write more than two or three letters in one sitting. If you try to do more they'll end up sounding rushed and too similar.
... forget to let them know what sort of candy you like so they can repay you. Just kidding. Gift cards are best. Duh.*
Obviously this would have to be adjusted if you're writing for a colleague or some other sort of situation. In a lot of instances letters can be used in tie-breaking sorts of situations, so you really do want to make sure that you're doing everything you can to help your applicant.
*I'm kidding, okay? It's not like I have a tip jar out on my desk. Although, slightly off-topic, when I went to pay my dentist ALMOST $1,000 the other day I paused, looking for the tip line on the receipt. Sleep, please?