The English language is dependent for meaning on sentence order. (Other languages depend more on other markers, like case and inflection.) If I have an ungrammatical messed-case sentence like:
Me and him went to the store.
You know what that means, even if "me and him" are actually the objective case—they are in the subject position (before the verb), so you don't have a moment's confusion. So think about using the order of the sentence to convey your meaning and propel the reader forward.
Pacing is about far more than sentences, of course. But I can offer some techniques that will help you use sentences to move the reader in the direction you want the readers to go-- towards greater understanding, towards some important realization, past some important clue you want to put there but don't necessarily want them to notice. If you want to propel the reader towards something or to create suspense, you can aid in this effort with sentence construction.
Let's start with a couple terms from Prof Brooks Landon:
The cumulative sentence-- the purpose of this is to accumulate and transfer information in the most clear (and in fiction, "good" -- however you define "good" in this context-- exciting or elegant or entertaining or amusing) way. Usually this means a clear and direct main clause (what Landon calls "the kernel") and modifying elements, and the meaning comes from the cumulative effect-- the gathering of the different bits of information. What's important to note is-- you do not have to put all these bits of information into the main clause, nothing like:
This beautiful exotic vista, a variegated Persian carpet, covered the purple-gray ancient mountain ridge and rustic deep valleys.
You can have a direct main clause and add modifiers as needed.
The suspensive sentence—the purpose of this sentence is to withhold information, at least that most important moment to the end, creating tension, suspense, and (maybe) comedy. So it might start with a main clause, but keep building with modifiers, and end with a bang, like:
She dressed carefully, sliding into a silver slip dress, pulling on shimmering thigh-high stockings, stepping into high heels, and slipping her derringer into her bra.
Both of these sentence patterns are useful in improving pacing, but you can probably see that they're probably each appropriate in different situations. The cumulative sentence is great for conveying information, for getting a lot across quickly. And the suspensive sentence is great for withholding information as in a punchline, so it might be good to set up the conflict (why does she need a gun if she's going to a party?).
Now let me talk about a few things that will make the sentence propel the reader forward. First, as Landon points out, the sentence should usually aim forward in time. To me, that means –
1) Don't puzzle the reader with indicators that take them out of time, like "before" and "two weeks earlier" and "once". As much as possible, make the sentence proceed chronologically when you want the reader to get through the scene and read on without stopping. Here's what I mean:
Before he entered the room, John knocked and waited for her to ask him to come in.
See how the "before" could confuse the reader? It's not chronological. It sends the reader back:
1) He entered.
2) No, first he knocked and waited.
3) Did he enter then? We don't know for sure. ("Before" sends the reader back to the beginning to find the end action, plus you don't actually SAY he entered.) but AFTER is usually chronological--
After he knocked, he heard her call for him to come in, and he entered.
(Okay, boring, but you see the chronology follows the sequence of the action.) When the sentence moves in the forward direction, it propels the reader forward. So only use the backward sentence construction which goes the opposite time-direction when you have a good reason—don't do it accidentally. (Flashbacks of any length are likely to send the reader backwards, and so interfere with that propulsion forward.)
2) Try and make that main clause pretty direct. As long as the main clause is clear, you can add a lot on (the cumulative sentence), modifying phrases and adjectives and adverbs which add to the meaning, without confusing the reader. What's a clause—a subject and verb combination. The main clause usually contains the actor and action of the sentence, the he or she, the main thing that happens. This does NOT have to be the most important thing (the derringer in the bra!), but maybe it can identify the central person or thing:
She dressed carefully,
If this is clear, then the reader won't stop and wonder and go back and try to figure the sentence out. Clarity is important to pacing (you don't want the reader stopping), and that's especially important in that main clause.
3) When a sentence is designated "too complicated" or "too long," the problem isn't usually number of words, but a burying or breaking of that main clause. So if you want to be clear, if you want to propel the reader through the scene, simplify not the whole sentence but that main clause. The first way to "clear" is to make sure that there's little separating the subject (which can be a –phrase-, not just a word) from the verb. So be careful of "appositives," which are modifying phrases that follow the noun, like:
Her older brother, a long-time resident of Paducah, edited the local newspaper.
Appositives break up the flow of the sentence by keeping the actor (subject) from the action (verb). A clearer sentence might put the modifier first:
A long-time resident of Paducah, her older brother edited the local newspaper.
4) The main clause doesn't have to start the sentence, but there's no doubt that sentences that start close to the main clause have greater directness and propulsion. The reader doesn't have to read far to know what is going on, what is happening. So be really careful about putting much in front of it. The biggest mark of an unsophisticated writer, I find—the thing that alerts an editor or agent to the "newbie" status—is a lot of introductory participial phrases. What works in front of the subject? Markers of time and space, like "In April," or "A few miles from town." The reader might need that information to understand the rest of the sentence. (Also see dependent clauses below.)
5) So put the "cumulative information" in phrases and clauses after the main clause, and see how that impels the reader forward. You can actually have very long sentences without sacrificing much clarity if you have a main clause followed by modifying phrases, like:
He never thought he'd love someone like her, a reform-school graduate, a convicted felon, an orphan, a wild child with wild hair and a crooked grin, someone who had never been loved before.
No matter how many modifiers you add on there, the reader can follow—but the modifiers probably all ought to modify the same word (here it's "her"). And with the main clause first, the reader knows what the modifiers mean. What would the impact be if the modifiers came first:
A reform-school graduate, a convicted felon, an orphan, a wild child with wild hair and a crooked grin, someone who had never been loved before: He never thought he'd love someone like her.
(The colon is needed because otherwise it might seem that HE is the reform-school graduate. Trust me on this... it's too complicated to explain.
6) Sometimes in suspensive sentences, the main clause will have that "most important thing" and be at the end—but the suspensive sentence shouldn't be your primary construction. It should be reserved for when you want to have that "punchline," at the end of a passage or a scene, maybe. It's more of a conclusory construction. Most of your sentences should probably front-load the important element, in the main clause.
7) Now the main clause is going to be an independent clause (can be a sentence on its own). That'll have the main action. But what if you have more than one action and subject? The compound sentence combines TWO independent clauses (with a comma and a conjunction), like "He spent most of his time at work, but sometimes she enticed him away." When does that work? Well, the compound sentence is an effective way to accumulate not just information but action (or feeling), because it makes room for two clauses of pretty much equal importance. Just don't clutter this up with introductory phrases or interruptions (like appositives). But trailing modifiers (at the end) are probably going to help the reader "surf" onward.
8) Dependent clauses (When I was young) can be useful for imbedding lesser but still important action or feeling in a sentence, and won't detract much from the clarity of the main clause. Dependent clauses can be either before or after the main clause, and you might try it both ways to see which one works best:
When I was young, I was in a punk band called Latex Novelties.
I was in a punk band called Latex Novelties when I was young.
(Theresa, remember that band? Used to play at the Jazz Kitchen.)
With "time" dependent clauses (when, since, after), I usually put the dependent clause first—the reader should probably know ahead of time when this action takes place, so she doesn't think, "Alicia was in a punk band last year?" (I was never actually IN a punk band. :)
9) While dependent clauses (like this one :) are useful and don't slow the sentence down much, "relative clauses" really do over-complicate and decrease the pace. Technical stuff here—dependent clauses usually modify the verb (they say when or how something happened, so I was (when I was young) in a punk band. But relative clauses (the ones that begin with "that or which or who" usually) are adjectival, modifying the noun:
Huckleberry Tucson, who was the sheriff of Mullenberg County, dug deep into the dirt, looking for clues.
The terror, which grew with every moment, felt like an octopus strangling her life.
The problem with adjectival modifiers, especially these relative clauses, is that they are "bound" modifiers, stuck to the noun—to be grammatically correct, they should be adjacent to the noun... and yet, unlike dependent clauses, they can't go in front of the noun—they can't actually be moved at all:
Who was the sheriff of Mullenberg County, Huckleberry Tucson dug deep
into the dirt, looking for clues.
I bet you've already figured out how to make that a propulsive, working sentence—reduce the clause into a phrase by getting rid of the clause subject/verb==
Mullenberg County sheriff Huckleberry Tucson dug deep into the dirt, looking for clues.
See how that propels—the sheriff... who is that? The question is answered as soon as it's posed—Huck. The cumulative sentence anticipates and answers the reader's immediate questions.
If you find these relative clauses, reduce!! Relative clauses are (because they restrict—that is the prime function of relative clauses) legalistic, and you really don't want your scene to sound like a legal brief. :)
10) Don't forget the one-word modifiers, the adjectives and the adverbs. These can truly change the tone of your sentence and sharpen the picture or character for your reader. So:
The old man wandered aimlessly through the deserted streets of town.
Some writers will say never to use adjectives and adverbs, but you don't have to go with that. The result is often either minimalist prose (not appropriate for so many stories) or far more complex modifiers (like phrases and clauses) which can load down the sentence. Always try to reduce, from clause to phrase to word, but experiment— go with what feels and sounds best while giving the reader the clearest meaning.
11) There is no reason to feel like you have to cram too much into one sentence. That's a good reason for starting with that main clause, because then you can make sure that everything that follows relates specifically to something in that main clause. If a sentence feels too long and complicated, go through and see if it will be easier to read as TWO sentences. Experiment!
12) In the end, what matters is meaning. The sentence should MEAN the right thing. If you aim first to make the sentence mean something, then you can embroider or add or subtract. But don't think in terms of words here, but meaning. Too many writers are obsessed with "hiding" the subject of a sentence, putting something, anything, in front of that "he". Yes, it's nice to vary sentence openings—but that is not your purpose here. Get the meaning right, and think about what the reader needs to know and when the reader needs to know in what order—and your sentences will be varied.
To keep the reader reading—to keep the pacing going—make your sentences clear, purposeful, and informative. Also make them propulsive, heading in the direction of the end of the scene. Try not to make the reader circle back or read over—make them keep reading by aiming forward and not back or around.