Plausible plotting starts with cause and effect. Make sure each step in your plot has a causative event, and one of more effects. Character actions should be caused by some motivation, and should have some effect on the plot.
Your protagonist should save the day (or destroy it). Protagonist is the “first actor”, the character most active in the story. Most importantly, he should be the one who resolves the conflict in the climactic scene. No one else should solve the mystery, or discover the secret, or arrive just in time to save the day. The plot should force the protagonist to make choices and take actions, and the course of plot events should change in response to those choices and actions.
Give the protagonist a goal, then take it away. The goal-driven protagonist is an active protagonist, but if you just let the protagonist achieve his goal, you’ll have a linear or two-dimensional plot. Have him lose the goal, or sacrifice it, or achieve it and realize he doesn’t really want it, and you’ll add the complication that makes this a real story.
The point of plot is change. The events should cause a change in the protagonist’s inner life, to trade her original goal for a more worthy one, to face a personal issue she’s ignored before, or to resolve a longstanding internal conflict.
Lead readers to the story, but don’t drag them. Set up your opening scenes so readers are led to ask story questions like “Who killed the film director?” or “What will happen to John and Sue’s love when Sue learns that John has been lying to her?” The posing of the questions, and the desire to find the answers, keeps readers turning pages. That’s called narrative drive. The story question is also an excellent tool to help the writer keep on track.
Make the internal come external. Explore your protagonist’s internal needs and values, and consider, how will this affect her actions? The external events will cause internal change… and the internal change will cause new external events.
Twist a cliché. Do something new with the tried and the true. Use the clichéd plot not as something to reproduce faithfully, but as a classic human drama to explore in a new way. Show the human depth under the stereotype: the blonde bombshell who walks into the private eye’s office is worried because her elderly neighbor won’t answer the door.
Coincidence kills plausibility. Don’t let a one-in-a-million event rescue your protagonist from trouble, or readers will stop believing that this person is truly affecting the course of events.
“Exposition is ammunition.” Tell the readers what they need to know, but only when they need to know it, and in the most powerful way. Make them beg for it. An essential question for all plots, but especially mystery/suspense plots, is “What should the readers know, and when should they know it?” Ask that every time you’re set to impart some extra information about the characters or events. Don’t tell so much so early that the reader has no reason to keep on reading.
Less is more. Don’t dilute the power of your story by layering on too many conflicts and motivations, or featuring too many secondary characters and viewpoints. Instead, focus on strengthening what you have.
Center each scene. Build it around some irrevocable event that changes the plot, and your pacing problems will vanish; readers won’t be able to skip because they’ll miss something important.
Find the excitement in every scene. Aim for the strongest, most dramatic events that are plausible within the world of your plot and your characters. For example, your heroine breaking in to an office and reading a file is more dramatic than her just overhearing the same information– but use this only if your heroine is the sort who would, under these extreme circumstances, break in to an office.
Always go back to character. The plot should show how these particular people with these particular strengths and values and conflicts react under stress or when pursuing a goal. You’ll lose readers as soon as they sense you’re forcing your characters to behave in a way that fits the plot instead of their personalities and needs.