Organize your ideas in an orderly and coherent way.
If your work is a sort of rambling, disorganized arrangement of 'thoughts,' then the reader will soon get lost, give up, and put it down.
When we get lost, we get frustrated, and lose attention. So, it is important to give the reader some idea of where you are going (you normally do this in the introduction), and then take them there in an orderly way.
So, how do we take our reader in 'an orderly way?' We organize our work into manageable chunks of united thoughts. In other words, paragraphs.
The point of a paragraph is to break our ideas down into organized, manageable sections. So, good paragraph structure goes a long way in keeping our reader's attention.
Paragraphs should, then, provide the reader with landmarks. Each paragraph begins with a new thought (or new development). Make sure to keep your paragraphs manageable. When a reader sees a paragraph that goes on for 10 pages, they despair, and wonder when they will get a mental break.