Turn the power off, hit the rocker switch on the PSU, unplug it and then drain whatever residual power there may be left by hitting the power button on more time before working on a computer. It only takes a couple of seconds. The only way you could get shocked after that is if you for some reason decide open a PSU to poke at the capacitors inside.
I don't remember how many computers I've worked on but I've never been shocked by one because I'm not an idiot.