I know several people who do decent workshops on plot, and I can name more good books about building your plot than any other facet of writing fiction, but there's one idea almost everyone has trouble grasping. In fact, only two of the eight or nine books I often cite even mention it.
Almost everyone understands that a plot is the stuff that happens to or around your protagonist, and most of them understand the idea of cause and effect. Most of them grasp structure and increasing tension, too. But making someone see that his or her premise needs more focus or oomph is hard. Maybe that's because everyone knows it when he sees it, but it's hard to define except by example.
Every story has a concept and premise, but unless it captures the reader's attention, the story won't sell. In fact, it won't even be read.
A concept is simply an idea. It can be a setting, a character, a story line, an imaginary world or practically anything else. But it has to develop into a premise, and that's tricky. The premise usually involves the "what if..." idea, the thing that "goes wrong." Michael Crichton's concept for Jurassic Park is that you can use the DNA from fossils to clone prehistoric dinosaurs. His premise builds on that: What if those dinosaurs get out of control and start eating people?
From that simple but specific foundation, you can build your plot because you have a conflict, setting and characters. You also have the beginning of your elevator pitch to an agent or editor. It even gives you a head start on your cover copy, which I always find hard to write.
Two of my books use Roller Derby as a loose concept. The premise of one of them is that a disgraced police officer finds redemption by protecting a group of women who help victims of domestic abuse, and, by extension, help themselves. That's more specific. More importantly, it helps me determine what will happen in the story. There will be roller derby and there will be at least one character who is being abused. The cop will help her. Sure, other things will happen, too, but that's the foundation.
Chicks on wheels, dirty deals, and everything you never dared ask about roller derby. Suspended after a "questionable" shooting, Hartford cop Tracy "Trash" Hendrix hires on to protect the local skaters from vandals while they prepare for a match to fund a women's shelter. He suspects a skater's ex-boyfriend, but the guy has an alibi when that shelter gets torched--and an even better one when he turns up dead. Then a skater is killed in a drive-by, and Hendrix knows someone plays rougher than the roller girls. Unless he can figure out who it is before the match begins, the wheels really will come off.
The fire and the drive-by aren't in the original idea, but they grew out of it and raise the stakes.
Your premise has to generate conflict, and this one does. In my case, that matters because my thought process is far from linear. I can come up with dialogue or character traits on demand, but plot is hard. That's why I need a concrete--but flexible--concept I can turn into a premise. And it needs to promise the reader something she or he hasn't seen before.
Most people stare at me when I tell them there are currently seven women's roller derby teams in Connecticut, but it works. I self-published The Whammer Jammers in 2011. Two weeks ago, I sold out every copy I brought with me to an event because people still want to hear about it.
That's your ultimate test.