fjm posted a great visual explanation of how to use a circle to plot. She talks about how in crime fiction, you have two stories going on at once, really--the detective solving the crime, and the story of how the crime happened.

To illustrate it to students, she explains:

Draw a circle open at the top. The end on the right label Beginning, the end on the left lable End.
Plot your *crime* around the circle.

[...]Plot your *investigation* from where your investigator comes in. If we're talking a dead body here, then it may be at End, the body on the carpet. From there, take the line over to an event on the other side of the circle. Then over to another arbitrary event, repeat until you've covered two thirds of the points you put around the circle.

You now have
a) a non linear investigation which uncovers the evidence in an order different to the actual events of the crime.
b) you've left enough "untold" information to make your detective look smart.

I really like this. I think it can apply to any story in which a mystery occurs, true crime or fantasy. I think I'm going to start sharing this with authors whose plots include mystery.