A trick I picked up from working at Zenith Music is to periodically use Bore Oil on a rosewood fingerboard. Bore oil is used to seal the inside chamber of wooden clarinets from excess moisture, and a small amount rubbed into a rosewood fingerboard brings it up a treat and protects it. Some folk think lemon oil can be a bit harsh, but I have never had a problem with the Gilly Stephenson's Lemon Oil.
El