Agreed with do what feels right - in general. But there are things you can do to improve comfort and performance.

LOWER BODY:

If you get your hip angle right, that will set you up for maximum power/comfort. Here's a fairly simple way to do that (easier demonstrated on the bike so ask if this doesn't make sense):

1. While on a flat road (or better yet a stationary trainer) coast and stand with your feel at the 3 and 9 position (level).

2. Lower your nose to your stem.

3. Sit down.

This will give you the best pelvic tilt (though it may take some getting used to) for riding, in general. If this causes frontal soft tissue pain, check your saddle to ensure that it is level and not tilted back.

UPPER BODY:

As for your upper body, keep it relaxed always. Bend your elbows. Hand position should be relaxed and wrists neutral - as if you are shaking hands with the bike. Shoulders down (not acting as ear muffs).

GENERAL:

Smile. It is almost impossible to hold a tense riding position if you are smiling.