The most efficient thing for me has been Illustrator w/ wireframe symbols. It's not so low fidelity that I can't envision the end products, additionally I enjoy the control vs. mockup tools like Balsamiq.
There some good wireframing resources for Illustrator and other programs in this Smashing Magazine post: http://www.smashingmagazine.com/2010/02/05/50-free-ui-and-we...