I am so proud of this (I'm sure others came up with it way before me, but I still came up with it all by myself!)

    //*
        normal path
    /*/
        debug path
    //*/
take away the the first slash to toggle the debug zone ON!

  #if 0
    // Normal path
  #else
    // Debug path
  #endif
Only works with C and C++, though.

It's awesome in both senses! How readable has it become for you over time? Because on first glance and without syntax highlighting it's a head-scratcher.

With syntax highlighting I instantly know what it is :) But to be honest it's just for when I find myself toggling something a lot during development.. but still am too lazy to make a variable.