I'm unsure if this actually worked, or if I was one of the lucky ones. I'm average tech-wise, but took my time and read a lot of forums prior to installing KitKat.
Here's what I did, very simply. Hope it works for you too:
1. Deleted any unused / unwanted apps. Installed any updates available for the keepers.
2. Moved any apps stored on the SD card over to device memory. (I did this since the KitKat doesn't write files to SD card - figured this may make the new OS install with less glitches).
3. Installed and ran Clean Master (Clean Master - Free Optimizer - Android Apps on Google Play). This freed up over 5GB of space on the S3 device memory!
4. Ran KitKat installation.
5. Held breath.
6. Messed with a few settings and charged overnight.
So far for the past 5 hours this morning I have experienced no issues, and seemingly better battery life.
Good luck!