When you ask for a letter of recommendation, you should ask
if the faculty member can write you a strong letter
provide information about what the requester of the letter of recommendation is looking for.
provide any information that you think specifically should be mentioned
You may want to provide a resume
Remind faculty about what courses you had them with, any accomplishments you would like highlighted
Any deficiencies/weaknesses that the faculty member could directly address
provide a deadline–with at least two weeks lead time
There is more information about letters specific to graduate school.
Follow up with your recommender about the outcome.