dont forget to include the date that your resignation will be effective (last day). make sure to give them at least 2 weeks (4 is better) advanced notice.
Dear Mr. Wonka,
I am writing to tender my resignation as head Oompa Loompa. My last day will be September 1st, 2005. <insert reason if desired, but keep it short and tactful>
I enjoyed working for you and the chocolate factory. The experience I gained is invaluable.
If you have any questions or require assistance in the transition, please call me at (800) 555-1212.
Sincerely,
George Loompa