Let every sentence make the reader think. If it doesn't it should be rephrased or discarded. Writing should be entertaining. Every last sentence needs to contain one of these elements. A boring sentence can ruin a paragraph. A boring paragraph can ruin a chapter.
1. A word that makes the reader think. 2. An interesting comparison, metaphor, or parallel. 3. A sentence that moves the plot forward. 4. Dialogue that speaks volumes.