Familiarize yourself with the position. Get a copy of the job description, if you can, and talk to the person you will recommend. If you know the intended recipient of the letter, ask them about the position as well. The more you know about the position, the better you will be able to tailor your letter to suit the needs of both parties.
How do I approach potential letter writers? First, make a list of professors and/or supervisors who will be your best advocates. Then, set up an appointment to discuss your request in person. Do not make the request via email. Be prepared to articulate your interest and reasons for attending graduate school. Letters of recommendation are written strictly on a voluntary basis. The best approach is to ask potential letter writers if they are willing to write you a strong letter. If you sense reluctance or the answer is no, ask someone else.