2015 May 5: Although I believe that this butterfly might possibly be a Clouded Yellow Sulphur, I don't know much at all about how to ID a butterfly ...so I've always just taken the easy route and called them "yellow butterflies."
These butterflies have always been very hard for me to photograph, as they rarely slow down enough for me to get a good focus on them with my camera. When I saw this one, however, I was determined to get a photo; I chased him through a field wildly clicking my camera's shutter button.
He stayed still just long enough for me to get this photo.
Maybe try a small aperture and manual focus with the fastest shutter speed you can get away with. Butter flies have always been a challenge for me as well. You're right, they just don't stay still long enough.