I could never get my top-down mountains looking ridged enough until RobA gave me this cool and fairly simple work around. Use a b/w height map and colour select to select an area corresponding to the area where your mountains really start to slope, making sure you use some feather (between 10 and 20). Then add another layer and apply a gradient to the still selected area. The gradient should be black/white with white at the points, using the shaped angular gradient setting. If you want steeper slobes you can alter the curve. Then make the layer with the gradient invisible. Then you select the layer with your terrain on it and apply a bump map, using the gradient layer as the bump. You should get fairly passable ridges.

Torq