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.
If anyone wants to work on their flash fiction writing skills, Holly Lisle offers a free 3-week online course. Don't worry if you're shy- you don't even turn anything in. She's written a ton of books and the course is good, in my opinion. The neat thing is it's structured so that you write five stories at once. She then recommends putting them up on Amazon as an e-book for 99 cents. And that fast, you're in the game! :)