The Hand_off. Finally, state your willingness to help out with the transition. You don't need to go into great detail (and definitely don't promise anything you can't deliver), but a couple of lines stating that you'll ensure a smooth wrap_up of your duties will show that you're in the game until the very end. Obviously, feel free to adjust this a bit based on your experience and your company culture, then submit it via the typical procedures at your company.
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).