Start by making your racquet takeback higher. If the ball is at your shoulder, your racquet should definitely be above your head in the takeback. Your still going to go high to low, but yes it is harder. I use a wrist release to really snap the racquet up to the ball. It's hard to hit through the ball this way, but it does give a helluva lot of topspin. And unlike a normal forehand where the racquet head is either below your hand or level with it when you make contact, with this shot, the racquet head will probably be above your hand. Because of this fact, you'll probably also get a good bit of sidespin in the shot. Think about it, once the racquet is above hand level, it is still moving upwards, but also to the side.
Maybe you could find a video of Fernando Gonzalez hitting a high forehand. He is good at this shot.