there is something everyone missed, that is, to pay the correct amount of money from a proper supplier for all cables, interconnects, crimps, crimp tools, wires, etc.
so
1) proper components, cat ratings, insulation, etc.
2) get the good fluke probe kit, think before measuring, ideally don't interact with circuit while its powered on
3) put indicator lights (neon) to make sure you know when something is energized
4) clean the work area, use a different outlet for the circuit under test then your equipment (don't plug it all into the same power strip), the main close calls I had were because of this, i.e. unplugging the wrong thing. This is when I suggest having two work areas, one for testing, the other for any disassembly, repairs, changes etc. So basically, ban the screw driver from being on the test bench ever, so you do your changes, move it to the test bench, test it, then take it back to the other bench to do anything (including stuff like changing breadboard components)
5) use fuses
6) clean tight grounds
7) good way to get zapped is to lean your hands on a chassis for stability with probes etc, the problem is usually that there is a fuse holder, power inlet, etc, with exposed wires there, then you can easily bridge the circuit (especially on large equipment). If you can, design so there is nothing on the side panels that is non insulated

follow peoples advice on isolation/gfic/etc, I am not going to get into that, it should be secondary protection from good practice and design
9) when destroying old equipment with cutting (i.e. you found a bad cable), drag it away from where you are working so you see both ends in plain sight and then cut it, don't do this while sitting at the bench after you were working for a long time
10) get tools with insulation, even the crappy rubber comfort grips can protect you a little bit (i.e. klein wire strippers), ditch the stuff with compromised handles for the pure mechanical work area (protects from #9 fuck ups). if you short it, throw it away after arcs occur where they are not supposed to, you have no idea whats going on there exactly
11) take time to get rid of fiddly stuff that you think is bad (i.e. if you clip to something and it keeps falling off, just stop until you figure out a reliable way, buy the right connector, some kind of helping hand, etc)... unless you are trying to get something done in the middle of a goauld attack.. its not battlefield repair
12) inspection, check for insulation damage, alligator clip alignment, equipment stability, etc
#7 and #9 are the ones that are most likely to get you IMO, with work fatigue, because we are often workahaulics