Sliding forward on the seat with periods of increased effort allows you to sit more over the bottom bracket with your legs over the pedals. You can then sit back into the saddle for a more comfortable ride when you are not going all out. I tend to do this on mine. If you are constantly on the front of the saddle it may be worth trying to adjust the fore-aft a bit or trying out a frame with a steeper seat tube to see if that helps?
There is an interesting article from Dave Moulton about frame geometry on this:
Sliding forward on the seat with periods of increased effort allows you to sit more over the bottom bracket with your legs over the pedals. You can then sit back into the saddle for a more comfortable ride when you are not going all out. I tend to do this on mine. If you are constantly on the front of the saddle it may be worth trying to adjust the fore-aft a bit or trying out a frame with a steeper seat tube to see if that helps?
There is an interesting article from Dave Moulton about frame geometry on this:
http://www.davemoultonregistry.com/Cycling%2011.13.1976%20Moulton%20Frame%20Article.pdf