As far as the Kindle store, the only way you can do this is to make the book free somewhere else, like Smashwords, and then wait and see if Amazon picks up on the $0.00 and makes your book free on their site. That is how folks on here have gotten their books to be free on Amazon, but it doesn't always happen, and when it does, it takes a while. Then you can raise the price back up on Smashwords, and wait for Amazon to change theirs again.
It's out of your control, unfortunately, but that's because Amazon does not want to spend money cataloging, advertising, and serving up downloads on books they make no money from. Sometimes I wonder why they don't make freeness an option, with authors picking up the tab for the download fee (as they do now with their 70% books). That seems fair to both. Maybe someday...
--Maria