This webpage contains advice for people asking for a letter of recommendation from me or someone like me. It was written quickly, and suggestions to improve it are very welcome. (A number of people have already contributed great suggestions before this page even went public!)
You likely have a lot of things on your mind right now, and this is the last thing you want to worry about, but this deals with a central part of your application.
Letter-writing is one of my most important duties.
The following statement is rather obvious, but a reasonable number of people seem not to have considered it: Please remember that it is in your interest to make your busy letter writer's job as easy as possible.
For example, I will likely write letters for about 40 or more people this year, most during the fall, around my other duties. (Number of people needing letters this academic year as of May 11, 2008: 50, not including letters for the MSRI program.)
Please give me as much notice as possible so I can write as detailed a letter as I would like. A month is reasonable. Two weeks may be pushing it if there isn't much flexibility in my schedule at that time. If you give me very little notice, your letter will necessarily be rushed, which is in no one's interest.
You want to help your recommender write as detailed a letter as possible. Here are things that would help me. Only some may apply to you. I will likely only start writing your letter once I have all the information I would like (as there is always someone else's letter I can write first).