With a valve tool.
http://www.wgcshop.com/pcart/shopper.php?itm=GD-TL-VK
OR
http://www.wgcshop.com/pcart/shopper...m=KSC-TOOLS-MK
Alternatively, if you have some scrap metal or and old/crappy screw driver and access to a grinder, you can make a toolbit that will fit.