Always start with a character.
I recently suggested this to one of my friends from Creative Writing class. I find it easiest to launch into a longer or more complex plot by first finding a character. He/she doesn’t need to be like-able, but should at least feel comfortable. I’ll choose a name – sometimes it changes – and start writing. I’m never sure if the book begins here, ends here, or if this scene takes place in the middle. I think the key to this strategy is to get as deep into the layers of the character as you can. Once you’re there, standing in their shoes, you can see what their problem is. There’s always a problem. That’s your plot.