I had a fascinating but lengthy prologue to this article which I've just cut because, you know, it was just me prolixing on about how cool I am. So let's just get to the goods instead.
Sometimes writers get stuck in the middle of their narratives. The story develops too quickly, for example, and your protagonist's journey from inciting incident to climax is over before you know it and your book is only 50,000 words long. Or, you simply don't know what to do with your characters on the way through Act Two (if you think in terms of a three-act structure). Anyway, there are three basic things you can do to make your plot more interesting during the middle of the story (and, incidentally, up your word count).*
The basic idea is to interrupt your protagonist's journey toward the climax. I will, alas, use examples from "Star Wars" because I doubt most of us share the same reading habits and references. Also, I get to be a geeky SW fan. Live with it.
Complications: These are things, people, or events that slow the protagonist's movement toward their goal. Luke being told by his uncle that he has to wait another year before applying to the Academy and leaving Tatooine is a complication.
Roadblocks: These are things, people, or events that stop the protagonist in his tracks. The Millenium Falcon being snagged by the Death Star is a roadblock.
Reversals: These are things, people, or events that turn the story on its head; the protagonist changes his worldview, his direction and often his goal. Luke's family being killed by the Imperial forces is a reversal.
All of these things are essentially problems the protagonist must solve. You can simply make them things that have happened to the protagonist, but that would be wrong. Why? Because a passive character to whom things happen but who does not himself act is boring. Rule Number One is that you do not bore your reader.
So, if your story is lagging, you give your protagonist problems to solve. Beginning writers tend to just throw these things at their characters in a long episodic middle stretch, and none of the problems are connected to the story as a whole. That's bad. All of your plot complications, roadblocks and reversals should grow out of things, persons or events that have appeared earlier in the story. Minor conflicts from Act One can loop back around and reappear as major conflicts in Act Two. This will give your story balance and shape and internal consistency.
Also, you might consider giving complications to your antagonist. How your villain (to sort of continue Michelle's post of yesterday about complex antagonists) deals with adversity might give us clues about his motivations and weaknesses, possibly foreshadowing the climax. Foreshadowing is teh roxor. Don't forget to use it. See above comments about events/items/people looping back around into the story.
So that's my advice today: if you find your story lagging or rambling in the middle, try to focus on the protagonist's goals. Point him at his goals and them have him trip, fall into a hole, come against a wall, or realize his basic assumptions are all wrong. Even if none of these ideas make it into your story, they could be useful brainstorming prompts that end up giving you ideas better than anything I could possibly offer.
*Does that all sound too formulaic and calculating and anti-art? I try to give pragmatic, nuts-and-bolts technical advice here. I don't know about art.