Cheat nature by forcing perfect symmetry
Open up your image (a landscape works well for this technique) and scoot up to select the Rectangular Marquee tool. We need to grab half of the image to flip. In our example here, the right side looks a little tipsy, so we’ll use the left. Click and drag with the tool to select your chosen half and then press Cmd/Ctrl+C.
Your selection will be on a new layer. It needs to flip over for the effect to work, so go to Edit>Transform>Flip Horizontal. Grab the Move tool from the Toolbar and click-drag to the right. This makes your image symmetrical.
Obviously this will create a perfectly symmetrical image and you will have areas such as puddles that can give the game away. Grab the Clone tool if you want to alter these or even use the Eraser to bring some detail back from the original half. We rather like the forced symmetry, so decided to stick with it. The only change we made was to turn the image into black and white for heightened drama.