There’s nothing that will help you go faster and feel more comfortable on the bike than a proper fit. Most famously popularized by Greg Lemond, getting and keeping your body in the most powerful and aerodynamic position has been proven to be the most effective way to improve performance and get better results.
In all my years of training and racing with a long list of knowledgeable and experienced coaches, I accumulated an understanding about bike fitting bit by bit and drop by drop. I learned so much about fitting from Lenny Preheim during my time with the Toga Bike Shop Pro Team. He often used George Hincapie as an example of what worked in the modern era. From all the hours I spent racing against the greatest riders in the world I developed a hunger to learn what worked for them and how it could be adapted for me.
In later years I absorbed vast amounts of knowledge from my great friend and old racing partner, Dr. Julian Dalby (also one of our Global Ambassadors). As a doctor and exercise physiologist, he taught me a lot about biomechanics, a great help in finding what works for each rider. I’m always focused on performance and my personal emphasis is always toward that.
The starting point for fitting a new bike is to first duplicate what the rider is used to. 
After in initial trial period on the new bike, we gradually dial-in the most powerful and comfortable position.
Today, everyone has access to a power meter, so all riders can visually see how changing one’s bike position effects output. Recently, I gave an experienced rider a bike fit and the resulting data were revealing. His five-minute power was up half a watt/kg. His one-minute power was up two watts/kg and as we keep refining his position, the improvements keep coming. Another rider estimated that his threshold power was up 15 watts immediately after we adjusted one parameter of his position and he’s a five-time national champion. He was amazed.