For years I used the ol' screwdriver method ( very carefully!) or the ol' reverse the bolt and coin method such as Morgan Fletcher suggested, but recently I purchased a Nitto stem tool off e-bay and boy oh boy what a difference. I had been planning on making my own tool using a photo of the Nitto tool as a model, but I couldn't have made it for as cheaply as I bought it.
My suggestion is keep your eyes peeled for one of these tools. As much as I despise e-bay (the old love hate thing) I've only been able to find them on e-bay
Jay Sexton
Sebastopol, CA