I try for as much realism and, if dealing with things that aren't actually real (e.g. fantasy races, Functional Magic
, advanced technology and such), logical consequences and internal consistency as possible.
So even if I've got people running around with cybernetic implants and replacement limbs I don't have the "Six Million Dollar Man" clusterfuck happening.
That extends to consequences of actions, so I don't have heroes speeding around the city streets, destroying store fronts and scattering pedestrians, then gunning down the bad guys, as anyone who pulled that crap in real life would be looking at some serious prison time - if they were smart enough to drop their guns when the cops ordered them to do so (and possible death at the hands of duly-appointed upholders of law and order if they weren't).
I even take it to the point of looking at what we have now and working out "logical" or "plausible" extrapolations and developments for any futuristic speculative fiction. What do we have now? What's being developed? What needs/requirements drive the research? What would be the first stage? What follows? Why? What plot points and story ideas are suggested by this?
Prosthetic limbs? Sure, people lose limbs and want good replacement. Eyes? Yep, them too. Built-in night vision and/or Thermograph? The military would want that. Would it be restricted? Probably not, but very expensive. Why would Joe Random Citizen want it? Very few would want/need it.