So, I'm a newbie, so you can take this for what it's worth - which is precious little. When I've felt that the plot of my current (and first) novel wasn't sharp enough, there have been three avenues of thought that have both been really productive for me and have led to great improvements in the story.
The first is to up the stakes. Look for ways to set things up so that your character has
more to lose. For instance, I went from a character risking damage to a cushy life in her wealthy parents' home to instead risking damage to a job she likes and needs, damage to her family's generations-old reputation, and the alienation of her entire circle of friends (this is more of an essential social lifeline than it might sound; it's 1951, and she's gay).
Next, try to make sure that all the
bad things that happen are a direct consequence of an action (or, less strong but sometimes okay, a decision not to act) on the part of a main character. For instance, I had my character's friends turning on her, blaming her for something that she didn't actually do. But this part of the book was limping along, and was a little boring. So I thought about it a lot, and eventually set it up so that she actually does do the thing she gets blamed for, albeit not on purpose; in fact, it happens when her attempt to
prevent it from happening backfires.
Finally, look at your minor characters and see if you can
combine them in a way that tightens up your story and ups the stakes. I found several exciting ways to do this in my novel. For instance, I rolled the man who wants to marry my main character (whom she does not want to marry) and her boss (a McCarthyite crusader pressuring her to snoop on her colleagues) into one character. It ups the stakes, creates a stronger and more central antagonist, and lends historical flavor (yay casual sexual harassment) all in one stroke.
I hope some of this is useful - like I said, I'm still learning how to do this myself.
