I work in Photoshop, without a plugin. I start with a line on a separate layer showing where I want the reflection to pivot about, and then work on the portrait layer. As in, I'll paint for a bit, maybe overlapping that diagonal, then copy the layer and rotate it 180. (I usually have to do a Select All then rotate, to get it to rotate to the right place.) I then adjust as necessary on the master portrait layer, then copy and check again. It's an iterative process, but with practice, it gets easier.
I thought about looking for a script/plugin, but I'm still working in Photoshop Elements 2, so I'm behind the tech curve and didn't think it worth trying to bludgeon when I can just learn to paint it up.