Perhaps something like this would be good:
Dear Ms. (her name), I am contacting you because you so kindly agreed to write a letter of recommendation for me. I have not yet received this letter and I am hoping that you will be able to find time in your busy schedule to prepare it for me. I know that your recommendation would be very helpful in my efforts to secure another position. Thank you again for all that you have done for me. Respectfully, (or Sincerely,)
I think it is best to be a bit more formal with authority figures, such as this, but perhaps another teacher will have a better point of view. I hope this gives you some ideas.