The following statement is rather obvious, but a reasonable number of people seem not to have considered it: it is in your interest to make your busy letter writer's job as easy as possible. I have more time to devote to writing a great letter if I don't have to spend as much time getting all the pieces together.
You should send me in one email at least two weeks for your first deadline (and the sooner the better, as that gives me more time to write the letter) a copy of everything you will submit with your application (cv, transcripts--unofficial is fine, essay, statement, research proposal, GRE scores etc.). In this same email, tell me what your earliest deadline is. In most cases, I will write one letter, and I will upload to everywhere at once before your first deadline. In the same email, you should tell me (even if you have told me before) all the places you are applying, and anything I need to know about where you are applying (no need to say anything about math grad schools or math academic jobs, but tell me about special programs, etc.) and who else is writing a letter for you. Please list the applications that will generate an email to me for uploading a letter separately (and marked) from those for which I need to email someone or go to some webpage myself (and for each of these give me the email or link). You should generate the automatic emails requesting the recommendations all at once (i.e. over a few hours) as much as possible, at least two weeks for your first deadline. You can usually do this by opening an account at each place you are applying without having to complete much of your application.
Feel free to let me know if there is anything you think would be good for me to address in your letter, or anything particularly positive about our mathematical interactions that you want to remind me of. Feel free to remind me if the deadline is getting close and I have not submitted the letter.