Asking You to Stay Beyond Your Notice. In this situation, you won't necessarily get the promise of more money or a better title__but your manager will ask you, perhaps as a personal favor, to stay a few additional weeks or months until the company is able to hire and train a replacement.
If You Want to Do More Than Just the Basics with Your Resignation Letter. You can always write more, and craft a longer resignation letter, but the truth is that it isn't necessary. You can add a portion of the note where you thank your manager or boss for the opportunities and time you have had together, or add that you hope to keep in touch, but in reality, if you really feel this way you probably should make an effort and find an opportunity to say these things in person (rather than getting an email).