Simply save the NEF after (or while) you make your edits. You can always revert to the original by deactivating the edit steps, or just click on the Version drop down and select "Original", however that will permanently delete your edit steps.
Better yet, make your edits, use the Version drop down to create a New Version with those steps. Then you can switch back and forth.
Versions are also a way to have the same image saved as a B&W and Color, for instance, or with different crops. Its much more space efficient than creating multiple NEF files, and easier to manage.
When I need a JPG or TIF of a particular version, simply call up that version and do a "Save As".