I remember seeing someone jot in the margin of one of my manuscripts: SVO.
I thought, "SVO?"
And it took a bit to figure out SVO stood for subject-verb-object. The problem with that prose was that every sentence was like every other sentence. This brings us to the next two writers mantras:
19 Vary your sentence structure.
20 If all your sentences start with the same first word, see the previous mantra.
Consider this example of bad writing:
Sally looked around. Sally saw the body. The body lay in a pool of blood. Sally grabbed her cell phone. A big man grabbed the cell phone from her.
Sure, some of you are mildly interested, but that last paragraph has all the piquancy of wallpaper paste. I'm old enough to remember using flour and water paste in first grade. Let's try again:
Looking around, Sally saw the body lying in a pool of blood. She grabbed her cell phone, but before she could dial emergency a big man grabbed it.
For one thing, I used fewer words, but conveyed the exact same content. Did the paragraph do a better job of catching your interest? I'm not claiming that was Hemingway, just that it was better.
If you're old enough to have used flour and water paste in first grade, you may have been forced to diagram sentences. It was the part of English class that sucked least for me, but I never got good enough to diagram the gnarliest of sentences. You won't have to diagram any sentences, you just have to make the poor schoolchild sweat who might get tasked with diagramming them.
The simplest way to vary your sentence structure is to combine two trite sentences into one. You'll see that I managed to combine three. Think of it like the game Boggle where you get more points for longer words: You get more points for combining more trite sentences. It may be a cheat, but I find gerund phrases useful. Just don't be stupid about it.
How will you know if you've got a problem with boring sentence structure?
The best telltale is Sally in the paragraph above. She started most of the sentences. If you find yourself saying she, she, she, instead of Sally, Sally, Sally, it's just as bad. Likewise saying Sally, she, she is a tip-off that you've got to diversify your grammatical structure.
You want to aim for clarity of expression and clear, concise sentences. But you can't be unbalanced either way. You can't go all Falkner on your reader and present her with a 2 page sentence to open your book. Unless you WANT your book to be tossed across the room like I did to Absolom, Absolom.
As with everything, don't overdo it. Moderation in all things. A sentence like, "Sally looked around," can be likened to the fellow in the back of the orchestra who stands there all evening with a hammer that he uses once to hit the triangle--ting. Conversely, some sentences can be like the one-man-band with a drum, banjo, accordion and who knows what all else lashed together.
Your prose should be somewhere in the middle.