The Tips in detail

 "When building up the final touches of a scene, change one object at a time."

The more objects you have in your scene, the more parameters there are you want to change until you get your final image. If you change several parameters a time, it becomes likely that you get lost in the parameter-jungle and forget which parameter you changed on which object. Furthermore it is harder to see interactions of certain parameters if you change a lot of them at a time. If this happens you are likely to spend your valuable time on finding out the parameters that messed-up the image.