Always ask for permission to use them as a reference, verbal or written first. And, don't forget to follow up to be sure they have written the letter . Lastly, send them a note of appreciation or a thank you card. The more specific to the job the better the letter is. If this is not practical a general letter that describes transferable skills relevant to your career goal will work fine for you.
Most students use three to six letters.
Sometimes multiple letters for different career goals are included in the file and the student provides the instructions on which to send. Career Services will keep your file for 5 years at which time it will be returned or destroyed. Your permanent academic record with the university is not affected. Any reference that is a year or more old may not be the best. It pays to keep your resume and letters of reference current if you plan to conduct another job search.