"Proper" cycling posture on a road bike with drop bars? Sometimes I feel a little pain in my lower back after cycling (nothing too long, mainly cycling around London for general commuting...5-10Km). It could be due to a number of reasons (I have a bad back anyway), but I would like to eliminate bad cycling posture as a possible cause.

I can only find one mention of cycling posture on SE, but it refers to flat bars:  Proper Posture on a flat bar road bike.

My bike is the correct size for me I believe, using the old "its just the right height for my crotch..Ouch!" measurement.

Does anyone have useful sources of information how to achieve the correct cycling posture?
Whatever works.  The main advantage of the standard drop bar is that it offers you multiple hand positions and multiple postures.  In a sprint, or driving into a headwind, riding fully on the drops cuts wind resistance, but for less intense riding one of the several hand position on the top bar or on the hoods is usually preferred.  And on long rides the ability to change positions from time to time is invaluable in terms of hand/arm/back comfort.

And there is no one "right" height for your bar.  If you're a real aggressive rider (and young and limber) then a lower bar position may be for you, but if you're somewhat more casual there's nothing wrong with raising the bar (so to speak).  And you may also want to consider adjusting your "reach" by changing the stem -- not everyone's torso is the same length.