"One way to get into that position is to imagine rolling forward on your saddle trying to put your belly-button on the top-tube. Then you need to slide back on the saddle to keep the weight on the back."

Danno's reply is great. A general comment is that too often people concentrate on the site of the pain rather than the cause. In this case, another couple of things you need to do are flatten your middle to upper back and push the crown of you head forward. A measure of success is that you will feel further forward over your stem when you acheive this for the same position on the seat. The ab and back strength mentioned by posters will allow you to maintain this position without massive pressure on your hands - indeed it should reduce the pressure as you are lower and better balanced. You should have a light grip on the bars not just when climbing but on the flats too. A last side effect of all this is you should be able to acheive better hamstring/glut engagement.