Consider this scenario:
The plot of your current WIP has run out of steam and you've exhausted all the tricks in your bag to get the creative juices flowing again...coffee, take a break/look at it with fresh eyes, brainstorm with a writing buddy. But, *sigh* nothing has worked. You feel helpless as you watch your WIP sink slowly into the quicksand of your mind.
We've all been there. I know I definitely have. But, I was lucky enough to go through a plot crisis during one of the NaNoWriMo Write-Ins. One of my very wise fellow WriMos came to my rescue with this brilliant snippet of a writing tip: "Merge this WIP story line with another WIP you have on the back burner."
What followed those words was a true "AH-HA!" moment. Endless possibilities for where I could now take the story line began bubbling up from the depths of my mind.
So when you have two or more plots that to you look dried up, put both of them in a mixing bowl, add coffee and fold together for a juicier plot.