How do I slow down the pace of my story? And please refrain from answering with "add more descriptions," because I'm already aware of that technique :). I'd like to see if there are even more ways to do this as well.
The answer to this question really depends on which scale you are talking about: do you want to slow down a paragraph? A scene? A chapter? The whole story?
You talked about the story in general in one of your comments, so I'm going to focus on that. Here are a few ideas:
If you're following the structure of scene and sequel by Dwight Swain (see this explanation by Randy Ingermanson), you probably want to put more emphasis on the sequels and less on your scenes. Give your characters plenty of time to reflect on the action that's happening and less actual action.
I don't think this works with every story, but maybe give it a shot and see what it does to your writing.
If your main plot is moving too fast, maybe you lack subplots. I'll go into more depth here because I have a gut feeling that this might be your problem.
Subplots don't only slow down the story because they drag the scenes of the story further apart. They also provide it with depth. You can use them to further explore your characters or, more importantly, their motivation (because that's relevant to the main plot). A full length novel should have 1 main plot and I'd say probably 1-3 subplots.
So, what defines as a subplot?
A plot, in general, is a sequence of cause and effect. Something happens, because of that something else happens, and because of that another thing happens. If two things happen and the first thing is not a cause of the second, they are likely not parts of the same plot. However, an action can be part of multiple plots simultaneously. If Jack and Jill go hunt down a murderer together, it can both advance the plot where they are investigating a crime and it can advance their relationship.
A subplot would therefore be a sequence of cause and effect that's distinct from the main plot of the story. The subplot itself should have a beginning, middle and end, just like the main plot. All the major plotting rules apply here, too.
How do I weave a suplot inty my story?
First, construct the subplot (as a series of scenes that are really causes and effects). You might want to make it contrast with the main story in atmosphere, (e.g. make it funny when the main plot it serious), perspective (show another point of view on the same thing or involve a character that disagrees with the protagonist) or what it's about, but make sure that it is still exploring the same theme (although it might show another aspect of it). Then identify the points where your main story and the subplot intersect. To actually weave them together and not make them seem like they are two individual stories that just co-exist side by side, you'll need them to share scenes. These will be scenes that advance both the subplot and the main plot (or two different subplots) etc.
Then, just put in the remaining scenes for your subplot in between the scenes for your other plot. For example, when time passes between two scenes in your main plot, it's a great idea to squeeze a subplot scene in between those two because it will enhance the feeling that actual time is passing. Or you can end a scene from your main plot with a cliffhanger and follow up with a subplot scene before returning to the main plot. Or, a subplot scene can simultaniously serve as a sequel to a main plot scene.
Subplots should stretch from the beginning of the book all the way to its end, just like the main plot. They should resolve usually either shortly before the climax (to maybe equip your main character with what they need to face the antagonist), at the same time as the main plot or afterwards in the Aftermath/Resolution of the story (kind of in a "Remember this? Yeah, I'm going to tie of that loose end, too" kind of way?).
To better understand subplots, I found this article on Novel Writing Help helpful.
You never want to slow down the pace of a story. Pace is everything. But pace is not about rushing to the exits. A pace is a comfortable speed at which to see all the scenery and experience everything that the journey has to offer. If the pace feels too rushed, this is not about the speed per se, it is about missing the key experiences of the journey.
The issue, therefore, is not pace, it is missing experience. But to fix that, you have to figure out what parts of the experience are missing. Different stories involve very different kinds of experience, so there is no blanket statement anyone can make to say that you must add 6 cups of this or two teaspoons of that.
Adding material that is not part of the key experience of the story will slow the pace, but it will fill the gaps with things that are boring and irrelevant. You have to find and show the stuff that really matters.
There can be two issues (that I can think of) that could lead your story to lack these essential details.
You don't actually have a story. You have a plot or a story idea. But stories are not merely the expression of an idea or a plot. Stories are experiences and they happen to individual people. Stories are real and particular. There are only a few plots. Boy meets girl a billion times. That is a plot, and the same plot is used a billion times. A story is when this particular boy meets that particular girl in this particular place and that particular time. Until it is concrete and personal, it is not a story.
You have a story, but you suffer from the curse of knowledge. You have not yet figured out what to say and what not to say to convey the concrete and personal experience of your story to the reader. There is no universal formula for this because the elements that make one story concrete and personal are not the same as those that make another story concrete and personal. You have to try to put yourself in the reader's place and imagine the effect you words would have on them. Extensive reading is the best way to train your ear and your sensibilities so that you can figure out what needs to be said to get your story across.
Another good choice is adding characters' background. Don't just explain what characters do, but also explain why. That also gives you the opportunity to insert flashbacks here and there, which will also add depth to characters. Fast-driven action usually tends to ignore the reasons behind the characters' actions and behavior, so adding them can be a good way to slow the pace down.
You can either add fluff of deepen the story.
Adding descriptions or reflection when your story does not need them, or events, or dialogues that do not add anything to the plot is "fluff", and I would advise against it. If you have a good story without the fluff, you should be proud of it.
Your style maybe is too dry and concise (Veni, vidi, vici), but this is a different problem, and I hope your critics can elaborate on their opinion.
Deepening the story is different from adding the fluff. For example, adding character's background is usually relevant to the plot. Creating subplots can also make the story longer and more interesting, as long as they do not overshadow the main plot. You can invent a whole new plot points, for example, instead of going straight from point A to point B, your character has a wardrobe malfunction along the way, and is in a desperate need of a fix.
I assume you are meaning that you feel the action in your story is too much, that it isn’t engaging or it is overwhelming the reader. Sort of like a modern 007 movie, where an intense amount of things happen with lots of tension and suspense, but not much story, or the plot gets misplaced.
You can break up the action with flash backs and flash forwards in the character’s life. I believe its a difficult technique to use successful. You can shift laterally at cliff hangers in the action and show another character or characters working on the next part of the story or waiting for information the character going through the too much action is trying get —- as in Where is 007. He’s so unreliable. While 007 is flying across the city in a jet pack trying to evade a heat seeking missile.