The most common / ergonomic way to go is to line up the flats(top) of the bars with the flats of the hoods as much as possible, yielding a large flat zone for the hands. This zone should be close to level with the ground (some like it slighlty up). If you have to rotate the bars back farther than 5-10 degrees or so, your reach is too long and a shorter stem should rectify it - if this is an option.

Something like this: