I really, really loved the Rose Metal Press Field Guide to Writing Flash Fiction. 25 experienced flash fiction writers/teachers each wrote an essay about an aspect of flash fiction writing. Each is then followed by an example story from that writer as well as an assignment. By the time you get done, you'll have a couple dozen flash stories and know lots of neat tricks too.
http://www.amazon.com/Metal-Press-Field-Writing-Fiction/dp/0978984862
http://www.amazon.com/Metal-Press-Field-Writing-Fiction/dp/0978984862