Here are some ideas...
1) send a letter or email updating your current situation, "I am on a new rotation..." or "I am planning on doing a rotation..." and nicely remind them by repeating that you appreciate them taking the time to write your letter.
2) call the office and speak with the secretary/coordinator and gently remind to remind the doctor via a note.
After 4 weeks and if you dont hear anything.. ask the secretary/coordinator if they got in contact with the doctor, if they say yes..just wait longer. If they say no then call or meet with the doctor directly and just ask, "Hi, I am aware that you are busy but I was wondering if you had the opportunity to write the letter?"
One letter writer took alomst 3 months to write the letter for me. I just wrote a letter or email every 4 weeks or so.
If you have not does so, you can also give them a packet that contains a prestamped envelope and the cover letter and CV with a note attached that states you appreciate the letter... This makes it easy for them and more likely to write it quicker.