The plot forms the underlying structure of your story, and if that structure is weak, the story will be weak as well. Many times it's hard for the writer to see the underlying structure of his own story, and this makes it impossible for the writer to judge whether this structure is effective and whether it is the best possible structure for the story.
One powerful method of revealing the structure of a story is to outline it. The outline reveals the bones of the story like an X-ray. You may outline a story before you write it, creating the framework of bones and then building on it, or you may write a rough draft of the story first and then outline it, revealing the underlying framework and making necessary adjustments.
Here's how to outline.
Number each scene in the story. Start with the first scene, number one, and summarize what happens in this scene. Focus on how the plot is advanced. Below your summary, answer the following questions about this scene:
- Have any conflicts been introduced?
- Have any previously existing conflicts been resolved?
- Have any conflicts grown in complexity or evolved in unexpected ways?
- What new questions have I raised in the reader's mind?
- What previous questions have I answered?
- What is the reader thinking/feeling at this point?
- What/who does the reader care about and why?
- What is at stake here? Are the stakes higher or lower than in previous scenes?
- What is generating suspense for the reader? Is this the correct level of suspense for this scene? Is the pace what it should be?
- Have I surprised the reader in this scene, and is the surprise believable?
- What does the main character want, has it changed and has the character changed? What choices does the main character have? What choices does he make?
- What does the reader now expect will happen in the next scene?
These questions all help to define the structure and nature of your plot and to reveal any weaknesses.
Now go to the next scene and do the same thing.
When you have outlined the entire story, look at the events described in your summaries and note where one event causes another. Connect these events with arrows. The strongest plots are created by cause/effect chains. This makes the story feel more like a row of dominoes falling over, unstoppable and inevitable, rather than a series of random occurrences arranged for the convenience of the author.
Now study the answers to the list of questions for all your scenes. Chart where conflicts are introduced and where they are resolved, where questions are raised and answered, how the reader's feelings evolve over the course of the story, whether suspense and surprise are used appropriately and effectively, and whether the main character changes over the course of the story. Check for loose ends. See whether you've set up reader's expectations so that surprises are powerful. Make sure the reader always has something to care/worry about. Draw a graph of the suspense level of the reader throughout the story. Look for any scenes or aspects of the plot that could be strengthened. Experiment with different possibilities. Decide which changes will most strengthen the story. Revise the outline to incorporate these changes.
Outlining is a very powerful tool, and the questions I've given you here are just a starting point. You can also create sub-outlines that deal with only a specific aspect of the plot, such as the evolution of the main character.
Now go try your hand at it!