Good Form consist of:
Head up, not hanging towards the floor.
Shoulders pressed downward, not scrunched towards the head.
The upper body is "Floating", the legs are doing the work.
Soft elbows with a bend and straight wrist with no pressure on them.
Tight abs and a strong lower back.
When you take control of the handlebars, you pivot at the hips without slouching.
Upper legs, lower legs, and feet all inline with each other.
Flat feet except for climbing at which point you will drop your heels.
And last, but not least, creating nice, smooth, circular motions with each pedal stroke.
A flat, neutral back. An engaged abdominal section throughout the ride. It's not easy.