If you need a letter of recommendation...

and you are an undergraduate who took a class from me within the last couple of years:

Email me. In you email:

Remind me who you are and what classes you took from me.
Tell me what kind of letter you need (grad school? med school? REU?)
Tell me when you need the letter by (if it is in two weeks or less I may not be able to do it).

If I agree to write you a letter (if I don't have anything good to say, I may decline to write) then I would need:

Your unofficial transcript
A list of places you are applying to with deadlines. I prefer plain text or pdf, no spreadsheets please.
(If you are applying to REUs through MathPrograms.org I may not need such a list since it's one upload. I still need to know when your earliest letter is due.)
If you, for any reason, make changes to the list you gave me, tell me ASAP!

Other things that may be helpful to me (this is less important than the transcript, the list of places or the deadlines):

statement of purpose
CV

If you an undergraduate and took a class from me more than a couple of years ago:

I probably don't remember you. I don't keep records for more than a year or two. So, no, I probably won't be able to write you a letter. And even if I could, it may not be that useful.

If you are a graduate student, a former graduate student or a postdoc:

Email me to remind me who you are and why you think I can write you a good letter. We'll take it from there. Keep in mind that if I don't have anything good to say, I may decline to write.