Wish I had a good answer.
Usually, when asked this, I end up looking like Bambi at the barrel-end of a shotgun. I stare, stammer, try to be witty. Stomp my hooves for a bit. But honestly? No clue.
I suppose I look at it as gathering my supper at a salad bar. I pick and pull and consider various leafy plot ideas, pile them on the plate of my mind, garnish them with what I hope is tasty style, top it off with a heaping dollop of personality. My dressing of preference is humor, heart, and horror. Steaming hot jalapenos of plot twists really set it off. I know, I know, it's an unlikely combination of flavors, but hey! It tastes great at the time, even if it burns when it leaves.
Finally, I let my palate of imagination hunger for the best. I demand shrimp. I know it's hidden away in the refrigerator of my mind somewhere.
Not good enough an answer?
Okay. Most of my books start with an image that strikes me. Out of the blue. Then I take it from there. For instance, I turned bored men sitting on the "husband bench" at the mall into a serial killer thriller/black comedy. I came up with a question, what might these men REALLY be doing there? They certainly didn't have many shopping bags nestled between their stretched out legs.
But what do I know?
Think I'll print this off, keep it in my pocket. Because, as I said, whenever anyone asks me where I get my ideas? I freeze, anything interesting or amusing completely gone the way of the leisure suit. This'll be my "get outta jail card."