I was always great at resignation letters, and I had the coolest one at my old bank. I was the only one who knew how to put the official bank stationary into the laser printer feeder. And 10 pt. Verdana...how much I love you!
I get falsely sentimental, explaining why I liked the job and some of my best times there, then mentioning why it is important for me to move on. In my case, it was for graduate school, a reason that they can understand.
Melon