From David Fiedler,
Your Guide to Bicycling.
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For maximum comfort and pedaling effeciency, you want your seat to be pretty much level, so that you can sit on it and pedal without having to consciously monitor where you are on the seat. Too much forward tilt, and you'll feel like you're sliding forward. Too much backward angle, and you won't be able to get any power and you'll have the sensation that you're slipping off the back. Both of these situations are distracting and uncomfortable.

When on a bike seat, your weight should be borne by the same spots on your rear that you feel underneath you when you sit upright on a hard firm surface. In addition to adjusting the tilt angle, you can also move the seat forward and backward in relation to the seat post. This will help make sure you're comfortably centering your weight in the right places.