Dear Mr./Ms. Bossname
I am writing to give you advance notice of my resignation. Due to (reason) it has become necessary for me to seek employment elsewhere. I plan to depart no later than (date).
I am giving you notice (amount of time) ahead of time because I would like to be of assistance in streamlining my transition away from the company. I feel that I can use this time to finish (project) and (project), but anything that will take longer than (amount of time) will have to be given to someone else.
I hope this does not unduly inconvenience you, and I thank you for your understanding in this matter.