As with anything, the more you do something the better and more efficient you get at it. With illustration, every project will help you with the next one in the sense that the more project experience you get the faster and more streamlined your work process will be the next time. Also, the more you draw the better you get. Having said that, however, each project is unique and presents a new set of challenges that need to be resolved. Experience will help in overcoming obstacles more efficiently.
Also, prior to making illustration my main profession my career was Graphic Design. I have found that my experience in this field has been a tremendous asset in the field of illustration. A successful picture book is a perfect marriage between the illustrations and the type. Each aspect is as important as the other. Having a solid grounding in design and good understanding of type, has been a huge benefit when composing my visuals and ultimately has made them more successful.