I guess my question is, how do you take into account “my character is smarter and better prepared than I ever could be, so she would have thought to take measures against this even though I, the player, did not” without having retcon mechanics like DMG and so-on?tzar1990
By letting the PCs win.
No, seriously, you let them just win. Here is how I decide if my PCs plans will succeed. I listen to them make the plan, and then that plan succeeds. I throw a handful of complications that players have to improvise around, but generally speaking I just let them outsmart the villains. I occasionally throw in a few rousing moments of NPC teeth-gnashing and fist shaking at the brilliance of the PCs undermining their carefully laid schemes.
It’s much like how I just throw some plot hooks into the air, watch as the PCs discuss them among themselves to try and figure out what the bad guys are up to, and then have the bad guys scheme be whatever the players cooked up.
Only game mechanics required are a poker face and a willingness to allow the players to succeed.