I am happy to write a letter for you if you are a current or former student of mine. Just remember that there is a fixed cost which is equivalent to a half day’s work for me: my letters are relatively long and detailed.
Short pro forma letters are essentially useless (take my word for this: I have been on the receiving end of letters of recommendation for close on to three decades). Include a recent CV, your transcript from the Graduate Institute (or another institution if relevant), a copy of a paper that you think is representative of your work (again, if this is relevant) as well as a short summary of your master’s thesis or PhD dissertation to jog my failing memory, if needed. A short three paragraph blurb about stuff you want me to include in the letter can also be useful, although I will of course rewrite things using my own secret sauce.
Give me enough time to write the letter, please! If the letter is going to several places, make sure that you include the precise email and any codes that need to be included in the message heading. If you are applying to a PhD program, please make sure to complete all of the information in the application materials such as my school address, office phone and title: having me complete various fields in your applications (especially if they run into the dozens) is not going to improve my mood….