Aimee Bender is the author of several novels, including An Invisible Sign of My Own, and collections of short fiction, including The Girl in the Flammable Skirt.
Set a timer for a slightly uncomfortable amount of time and just sit there. No leaving the desk. No internet, no email, no dishes, no phone. Get so bored you cannot stand it and have to do something. Sit through restlessness. (For the time—something longer than five minutes, but shorter than an hour.)
I have so many! Usually ones that offer structure in an almost absurd way. Ron Carlson has a great one about the alphabet—write a story in which every sentence begins with a consecutive letter of the alphabet, and somewhere in there include a 100 word sentence. The 100 word sentence is a tremendously good exercise unto itself.
I have received many gems, but one I return to very often came from a peer, a wonderful writer named Phil Hay, who said "write every day what you feel like writing that day and the writing will never be dutiful."