Dear …
After giving it great consideration, I have decided to tender my resignation as __(position)__ at __(Name of organization)__. I have been presented with an opportunity that will enable me to further my career in the area of __(type of area/field)__. It is the kind of opportunity that will allow me to grow in my field and I am looking forward to my new position and the challenges that await me.
My last day of employment will be __(date)__, which should allow you enough time to find a replacement. This will also give me plenty of time to finish those projects I am working on, before I turn the position over to my replacement. I would be happy to assist you with training the new incumbent during my remaining time with __(Name of organization)__.
My experience with __(Name of organization)__ has been very rewarding and I would like to thank you for the experience of having worked for such a fine company. I wish you and this company continued success.
Sincerely,