The only way to truly get an economically "correct" typing posture is to use a keyboard that actually has keys under your resting hands' fingers. That means using a keyboard that was designed to match the shape of hands, not the shape of a 19th century typewriter.

There is no ergonomical way to use a traditional keyboard. All you can do is pick your favorite compromise. Using a better layout (like Colemak) helps a lot, but it can't fix the whole problem.