I recall seeing a contributor's post on the "doodling" thread a while ago that was a compilation of chair concepts on post-its, and also more recently some doodles of Yo's that were done with a time limit.
Incidentally I was sketching last week with a blunt pencil. Not only did the pencil move quickly and fluidly, the limit of detail I could also sketch encouraged me to draw up a concept, add a few contour lines and shading and move on.
Might be worth experimenting with in future!
What other techniques do people use to speed up concept generation?
I'd love to post the outcome, I will do so in Feb 2016!