Position of your Hands on the Bike:
Avoid numbness in your fingers by moving the position of your hands often on the handlebar. Hold solid enough to keep your hands on to the handlebar at unexpected shocks but not so tight that the shock goes up in your arms. For similar safety reasons you should keep your thumbs around the handlebar rather than resting on the top.
Move hands down at the bending at steep descent riding and at high speeds and hold your hands on top of the brake levers when driving relaxed. At long increments hold hands on top of the handlebar and sit upright to open your chest helping you breathe. When you get up on a hill / mountain hold lightly on top of the brake levers and swing the bike from side to side in sync with your opposite pedal thread.
Position of your Arms on the Bike:
Keep the position of your elbows bent and relaxed to absorb shocks from the bike and prevent shock when you hit a bump. Keep your arms into your body and not torn out to the sides. This position of your arms on the bike is more compact and aerodynamic.
Position of your Head and Neck on the Bike:
Resist the temptation to hang your head when you go at high speed or become tired on your bike. It takes only a few seconds to be in a dangerous situation in the trafic. Change the position of head often an try to tilt it to the sides instead of holding tje head in the middle all the time. Change position of your hands on the handlebar so you reposition your upper body and give your neck a new angle.
Position of your Back on the Bike:
A flat back is defined as a stylish rider. Anatomy and flexibility has much to do with how flat the back you can get. But even a rounded back can be the best position for you if this position is demanded for you not to get back pain, just as Lance Armstrong do it. You can work your way towards a better position of your back on the bike by imagine you must touch the top tube with your navel. This position helps your hips to rotate forward. Do not ride this way all the time on your bike but it helps your back position down into a more aerodynamic position when you need it.
Position of your Upper Body on the Bike:
Do not be stiff and not flabby. Energy is wasted if you have a rocking motion from side to side with each pedal thread. Save energy for your pedal thread and try to keep that upper body steady for you to focus on the pedalling. Beware also that you do not sneak forward on your saddle and bend your shoulders. There is a tendency to do that when you increase speed. Switching to a higher gear and stand periodically in the pedals is a good change in the position on the bike to prevent stiffness in the hip and be seated correctly and relaxed back on the saddle.