Dear {insert name],
I hereby wish to give notice of my resignation from the post of [insert job title] as of [insert date]. An opportunity has been offered to me about which I have though long and hard and have decided it is time for me to move on to further my career.
I would like to thank you for your advice and guidance over the last [insert length of time], which I have always valued and will take with me. I have thoroughly enjoyed my time here and hope that our paths cross again in the future.
Yours sincerely,
Easy Groove.
(keep it short, to the point and no excuses or grovelling)