There are many ways to say it and there are many great articles about it. The only way to consistently improve a website (as defined by having visitors complete some kind of goal, such as filling out a form, purchasing, etc.) is to split test.
Split testing is where you have your existing webpage, then you take your knowledge and experience and change something in hope that it will convert better. Then you set up an experiment or test where some visitors see the original page and some see the new variation, and you gather statistics about how many people complete the goal your set. When you figure out which variation works best you drop the loser and use only the winning page. Then you set up a new test. This way you are constantly improving things with real data.
There’s no good way to predict what will work and what won’t without testing. Every audience and website are different. Data driving decision making is the only true way to consistently make conversion improvements.
Here’s a video I made for my clients showing how a split test works.