Does your plot have a beginning, middle and end? Do your scenes have the same? Do you have... CONFLICT!
Once upon a time there was
and that's it. Every choice you make after that should open the door to conflict and struggle and emotion and all those tasty veggies. Every character you choose to create, every character choice they make, every setting you choose, everything should have purpose and that purpose should be to move the story forward. You move forward by creating a reason for the story to move. Without conflict you have no reason to move.
This is explanation is getting a little complicated. Let's make it simple.
Conflict gives your character a reason to grow and move forward, thus progressing your story.
Stories are split up into three "Acts" and the two points between acts should represent your two main points of conflict. The first is known as rising tension and takes place between act one and two. In act one you've introduced your characters, your setting and the basic set up of your story. The height of that should be something that deepens the conflict and drives your character to action. An example would be in The Godfather, the end of act one would be when Vito Corleone gets shot (spoilers?).
Your second point is the climax which takes place between act two and three. This is your point of no return; everything is heightened. The climax in The Godfather would be the shooting at the baptism (again, spoilers?)
Act Three is mainly used for cleaning up the debris left behind by the climax. You come down off that high and create resolution. You don't have to wrap it all up in a neat ball but you do need to close it off a bit.
Stephen J. Cannell lectured on the Three Act Structure here
Now keep in mind this is just one way to structure a story. Granted it's the most common way but maybe you guys could think of some other ways to set up your story? How do you structure your plots? What are some tricks for making sure your conflict is engaging (and realistic; I can't stress realism enough)?