There is an instinct that writers develop after gaining some writing experience for how the rhythm of their sentences flow. Have you ever read a manuscript that seemed choppy...or worse, so long winded that the whole thing felt like one never-ending pile of mush? I have. And they were my own manuscripts.
At first I had no idea how to fix the problem, but after I got a little practice under my belt I realized that the length of my sentences had to be varied. They couldn't all be short or long, and they couldn't all have the same rhythm when they were read aloud. Like music, the language had to be interesting, broken apart by differing speeds,rhythms, and pauses.
Try reading your manuscript out loud to yourself. Does it sound short and choppy? Are all the sentences the same length? Try to vary the length of the sentences in each paragraph until they sound like they flow well when read aloud. Sometimes of course you may purposefully shorten or lengthen a few sentences in a row in order to create tension or another type of mood.
In any case, paying attention to the flow of your language will make your writing much more professional and enjoyable to read. After awhile, you will start to vary your sentence lengths automatically without even having to think about it.
"Confessions of a Completely Insane Mother"