cinghiale's method, which I assume is the same as popping the hinge, works for me.
I also stabilize the oyster by placing it on a mounded towel, and I place my nondominant hand over the upper shell to balance it.
I have shucked thousands of summertime oysters and clams. (Normally I'd link here to family clambake photos, but Flickr is destroying my memories with its new free-account limit.) Many bivalves are good fighters, so it may not be all your fault. Yet it's always easier for me with a great knife. If you have several good oyster knives but poor results, you may need to buy an even better knife.