This is based off an e-mail I sent recently to a student and someone suggested I post it here:
Asking for letters of recommendation
In general I, and others are happy to write letters of recommendation for people… it’s part of our jobs after all. However, there are some tips I would offer anyone soliciting letters at any stage of their career.
1) Don’t ask me for a letter only a few days before it’s due. This seems like such a simple concept but one that is violated so often.
2) If you ask me for a letter, you need to send a copy of your CV. No matter how long I’ve worked with you, there’s probably still information in there I didn’t know and this helps me write a letter that doesn’t sound like a form letter.
3) Send me a description of the program you’re applying for and why. Again, this helps me write a better letter and doesn’t force me to have to trawl the internet for information
4) Make it as easy as possible for me to write the letter! This is especially critical with professors. If the letter needs to be mailed, you should hand me a stamped, already addressed envelope so all I have to do is drop in a letter and throw it in the outgoing mail. If it’s an electronic form provide me with detailed instructions and links.
5) Don’t attempt to bribe me. I’m not kidding… for example once I got a handwritten request for a letter of recommendation along with $50. This is not a good idea!
(Updated with two more in May 2013)
6) Before asking a post-doc or a project scientist for a letter of recommendation make sure that you don’t actually need one from the professor.
7) Don’t list me as a reference for anything without at least asking first.
Jonathan Eisen's Lab 
Thanks David!! It’s always good hearing this type of advice.
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I’ve got one more to add:
6) Make sure that if you ask someone who is a postdoc or a project scientist for a letter that you don’t actually need a letter from a professor!
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And another: Don’t list me as a reference for a job without asking me first!
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This is great advice, I’d’ve thought including your CV would be like saying ‘i know you don’t really know what I do, so here’s a cheat sheet’, makes sense that it’d actually be a reasonable thing to do.
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I also think it’s important to ask the student to provide a list of bullet points for things they need you to talk about in the letter. This is important for two reasons. First, obviously, it makes it easier to write the letter. But more importantly, it means the student has to start thinking of the application as a complete package, designed to address the key strengths that the position calls for. A good application package uses each reference letter strategically to emphasize different skills, experiences and personality traits, so that the set of letters becomes more than the sum of its parts.
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