This video by Stephen Bourne ("One Minute Tennis Coach") offers a way to increase the whip of the forehand. As usual, Bourne provides a different take on tennis technique and with a clear explanation. In this video he shows that the professionals hit their forehands with more power than us amateurs because they're rotating around the left shoulder (if they're right-handed), not their right shoulder. He shows how they tuck their left (non-racket) hand in a way that causes their body to pivot around the left shoulder, not the right, thus increasing the length of the swing. (I don't know if this is the same reason mechanically but I've noticed that many professional football quarterbacks make the same motion with their non-throwing hand when they throw the ball.)
I recommend Bourne's YouTube channel. Many of his tips provide a unique -- and useful -- perspective.