When you’ve written and read enough, you begin to develop an inherent feel for the rhythm of prose. Amateur writers often write sentences without much thought to the rhythm and flow, resulting in a series of short sentences which begin to sound stiff and boring—or a series of exceptionally long complex sentences that also become too stiff and boring to follow.
What you want to do is have variation in your syntax. Alternate between long and short sentences, simple and complex. Make your prose almost like music, in that there’s rhythm and flow to a paragraph. It’s difficult to explain, but when you become experienced enough, you can recognize this rhythm when you read it. Often my first drafts don’t have rhythmic prose, but later while editing I can recognize the rhythm (or lack thereof) and re-write the sentences accordingly.
For instance, you don’t want repeated sentences that begin the same way, especially with the same pronoun, such as “I woke up. I ate breakfast. I went to work…” You need variation in your sentence structure in each paragraph, or else it become too repetitive and boring to the reader.
This may be a more high-level advanced writer’s tip. If you are just starting out you should focus primarily on story and character. Regardless, you shouldn’t think too much about the rhythm of your prose while writing the first draft—wait until the editing phase. Then you can try to make your prose sing.