Letters of Recommendation are a reflection of you. Students often don't understand that if they have no interaction with a professor outside of class, it is very difficult to get quality letters. If a professor doesn't get to know you, usually the letter will only reflect how well you did in a course (grade, rank in class). Compare this with a letter that also describes personal attributes such as social skills, verbal skills, determination, willingness to help others, analytical ability and one that comments on your career goals. These are the letters you're competing against. Do yourself a valuable service and make some effort to visit your professors and speak to them. It will dramatically improve the quality of your letters.
My personal policy is that I will only write letters for students who earn a strong B or higher in my course. Regardless of grade A or B, the strength of the letter can change if I can comment about the various things I mention in the paragraph above. My ability to do so, of course, is up to each individual student.
To request a letter, you may email me or stop by my office. I will need the address of where to send the letter and the information provided on the application form or packet.