Bike fit / correct riding position

From a seated position ( a chair is fine ). sit bolt upright, with your chest out. Proper stiff back, as if you're standing on an army parade ground. Then lean your torso forward, without bending your spine in the slightest. You'll find that the best way to achieve this lean is by rotating your pelvis forward.

You won't be able to maintain that stiff-backed position on the bike for long ( nor should you ), but it gives you a better feel of how you should be supporting your weight.