Bike position is about two factors: optimising performance, and avoiding injury.

One of the first guides is whether it 'feels' right. Often, if you have a doubt, its because you are not feeling comfortable. The flip side of that, is if you are not experiencing any injuries, its sometimes unwise to change things. If you do, make sure you make small changes in a stepwise manner i.e. change one thing at a time, and always make sure you have made a note of how your bike was BEFORE the change.

Changing your position because someone says you don't look right on a bike should always be backed up with an expert opinion. Its hard to gauge the 'best' position for a rider purely on what they look like.

The best place to go is normally your local bike shop: many now offer a thorough bike fitting process (especially if you are buying a bike from them).

Changing your position to optimise performance is a slightly more complicated process. Speed on a bike is based on your ability to produce power and the aerodynamics: as you would expect, there is a trade off: the 'wind cheating positions' are not often the most effective for power production.

• more aerodynamic positions (e.g. lower at the front) cause a smaller hip angle, meaning less range over which your muscles can work: result is a reduction in power
• Positions allowing more power (hip angle open) often result in a greater frontal area, and therefore more drag

SO, keep in mind, the aim is to go faster - it doesn't matter which side of the balance you use to get there.