Preserving a turtle shell requires at least a week's worth of time, some elbow grease and a clear polyurethane sealer. It is also important to make sure it is legal to own a turtle shell.
Make sure the turtle is completely gone from inside the shell. Place the shell on top of an ant hill for a week or bury it in the ground for a month to be sure. Place the shell in a solution of borax, water and salt for 1 hour, rinse and then soak in a solution of water and vinegar for 1 hour. Rinse the shell again and let it dry. Apply a coat of clear polyurethane sealer to preserve the shell.