This...
Get the Rotor Phasing dead on and the rotor tip to cap terminal radial distance as close as possible. No air gap=no spark to jump/no spark=no burn. Not possible due to the nature of the beast but make it as minimal as possible (more critical with a high energy system/multiple (MSD) sparks).
My Pro-billet MSD dizzy's phasing is so far off, I had to use the MSD adjustable rotor, and use almost the entire range of motion to get my rotor positioned correctly.
I discovered the problem when the metal tip of the standard MSD rotor melted and flew out of the end of the rotor. The heat that the large gap was causing a large spark jump inside the cap was intense. I was using a very high output coil too.
Once the phasing was corrected using the adjustable rotor, the rotors lasted much longer. I still saw signs of heat build up after a few thousand miles, so I swapped in a standard MSD canister coil and the discoloration from the heat pretty much went away.
Back in the late 70's when I worked at a tune up shop, we where trained to look at the bottom of all the rotors to see signs spark leakage on the bottom of the rotors. GM HEI distributors had a lot more power to transfer through the rotor that a standard points and condenser set-up, so the rotors on the HEI units would show voltage leakage in a year or two. Once the voltage made it past the rotors, it would cause the internal advance mechanism's to appear rusty with a rusty powder residue all over the internals. The mechanical advance would fail shortly afterwards requiring a replacement distributor.