Dear Mr./Ms. Last Name:
I regret to inform you that I am resigning from my position as Communications Assistance for the ABC Company. My last day of employment will be August 15, 2010.
Thank you for the support and the opportunities that you have provided me during the last several years. I have enjoyed my tenure with the company.
If I can be of any assistance during this transition, please let me know. I would be glad to help however I can.
Sincerely,
Your Signature
Your Typed Name