I am always happy to write letters of recommendation. Because of the number of requests that I get, I ask for a few things to help me keep track of them all...
A minimum of two weeks notice. Three is even better, particularly if I haven't yet written a letter for you. If it's less than this, talk to me to see if I can make an exception.
An email. For each letter you need clearly indicate:
Who/what I am writing the letter for (a link to the REU program or grant is sufficient). If it is a grant that aims to fund something in particular, tell me what that is.
A deadline.
Where I should send the letter. If a link will be sent to me via email, indicate that and tell me who to expect it from.
Materials. If you have them, also include any materials that might help me write the letter. This could be a CV, a draft personal statement, a grant proposal.
Also complete this form for each letter needed.
Follow up emails.
I will always confirm with you that I have submitted each letter. That means that as deadlines approach, it is your job to check in with me and make sure I'm on it. Don't hesitate to check in... I expect it and would rather you check in too often than I miss a deadline and hurt your chances.