What I do to keep a turkey warm after cooking it is to wrap the turkey in some tin foil and then keep it in the oven at 200 F.
Usually I serve the turkey within an hour of cooking it so I don't need to keep it warm for very long but the oven keeps the turkey warm without over cooking it and the tin foil helps hold the heat in.
You can also leave the turkey in the oven after cooking it and turn the oven on to the lowest temperature setting just to keep the turkey warm enough.
Or if you have an electric turkey roaster you can keep the turkey in their and keep it at a warm temperature.
Or after cooking the turkey immediately put some tin foil over it and then drape a towel over the tin foil to help hold the heat in the turkey.