Bouncing at high cadences

You do also have to concentrate on core stability, relaxed but solidly placed. imagine the gut area is made of stone, or lead and it is the centre of you and the bike as you pedal. upper body and arms should be relaxed but not floppy. It also helps if your bar height and reach is set up correctly to distribute your weight and balance optimally.

i'd echo this. pulling your stomach muscles in and holding your core helps make you hips feel lighter, if you rotate your hands away from the center of your bike you shoulders will drop in. then concentrate on flattening your back/stomach. i imagine my body is two seperate parts. waist down and waist up. concentrating on your core (inside your abs) and keeping your upper body in a good shape so you don't have to grip like a pompey whore allows your legs to flow that little bit more freely.

That and practice - i still ride with one leg at a time to improve my pedal stroke on each leg. i find it helps making the stoke fluid