Here is my policy, and advice, on recommendation letters. Please review this carefully before asking me for a recommendation. I enjoy writing letters for students, and following all of these guidelines will ensure that I can write you a strong letter.
My policy:
I cannot write you a letter if you ask me less than 2 weeks before the deadline. This is very standard practice (many of my colleagues require this kind of notice as well). Be vigilant about deadlines! Give me more than 2 weeks notice.
Some advice now:
Make sure I know you well: The best letters come from professors who know you well, either from a small seminar or from research or thesis experience you have had with the professor. If you don't know me in this way, you should not ask me, because my letter will be unavoidably more superficial than what you need. A letter from someone who really knows you is better than a letter from someone who you think would be a good person to have as a reference. If you're applying to graduate school, try to have two professors as references and a third from someone else who knows you well (like an employer, or a third prof if that is possible).
Send supporting materials: When you send me your request, please send me supporting materials. This includes:
Your updated CV (resume)
Grades and GRE scores (if applicable)
As polished-as-possible application essay drafts, or cover letter
Some suggestions (in a document or in the body of the email) for what you would like me to highlight in your application.
The essays help me learn more about your goals for the position or school. In your suggestions, you can include information about specific requirements or attributes of the job or school of which I should be aware while I'm writing the letter. Draw my attention to things that you've done or skills that you have that make you well suited for the place. Help me to help you.
Send everything in one email: It is very easy to lose track of many different pieces sent at different times--remember I write letters for over a dozen people every fall, and then more throughout the year.
Use a dossier service if you're applying to more than 2 places: For Princeton students, Princeton has a dossier service you can use, so that if you are applying to 12 jobs, I send the dossier my recommendation letter and the dossier sends it out to all 12 places. If you are not coming from Princeton, look into your own school's service or use a company like Interfolio, which many of my colleagues recommend.
Fill out online information for me: Most graduate school applications are online, but every school has a different custom form. It takes hours for me to fill out these forms, and this time is lengthened when I have to fill out my name, position, address, phone, etc., on each form. Please fill out the maximum amount of information about me so that I don't have to keep doing this.
Thanks!