Select the teachers that you will ask for letters of recommendation carefully. These need to be teachers who have seen you perform outstanding work, and preferably teach the subjects that you will be pursuing in academia or in your career.
Give your teacher plenty of advance notice — at least one month, but preferably six to eight weeks.
Provide the teacher with a typed business letter requesting the recommendation that includes deadline information and any special instructions — for example, certain topics you would like her to address. Include any forms issued by the university or employer that the teacher must fill out.
Provide a stamped envelope addressed to the school or place of employment if the letter needs to mailed directly. If the teacher will mail you the letter include a self-addressed, stamped envelope. Note that recommendation letters must be snail-mailed because they must contain actual signatures from the teachers.
Hand your teacher the request package in person, if possible. If that is not feasible, follow up with an email just to check that they received the request.
Be polite. Start the conversation by saying something like, "I've learned so much from you. Would you be able to write my letter of recommendation?" There's no need to go overboard with flattery — teachers are used to writing letters of recommendation and understand the importance of them.
Don't nag. Following up a week before the deadline is acceptable if you haven't heard anything from your teacher, but don't pester her. The most important thing is to be respectful of your teacher's busy schedule.