Good lord... I've finally got a handle on it. Thanks to trudatman and smozoma for all the advice. It turns out that I've picked up two bad habits that mess everything up. I tend to let my thumb slip off the up arrow on the D-pad, so when it feels like I'm rolling left to right, the puck doesn't actually move. Also, I often shoot too soon after making my last move, rather than waiting for the puck to actually switch sides, so I shoot right into the pads. I haven't broken myself of it just yet, but when I'm really paying attention, I can now actually pull off "the" move fairly often. What a relief.
Back to those video tutorials for a minute... is there some magic to the five-hole slapper, or is it just kind of a fluke? I've launched a whole lot of slapshots, and not once have I seen one go five-hole.