Below is a list of things to decrease the possibility of permaborking. Keep in mind that following this list will not 100% remove the danger of flashing bootloaders, but will increase the chances of a successful flash.
Flashing bootloaders is dangerous. They are one of the few things that has the possibility of hard bricking your phone. For that reason, here are some good things to do to prepare for flashing them.
Have a good, reliable data cable. Interruption of communication between your phone and pc will most likely cause a hard brick with bootloaders.
Use a usb port on the back of your computer (or #1 on a laptop). Communication, again.
Have a jig. Not much help if you hard brick, but if you soft brick its very nice to have.
I personally flash bootloaders from a good laptop with a fully charged battery. If there is a power outage, the battery in the laptop will have saved my phone.
When flashing bootloaders,or anything with Odin, don't touch your phone after you connect it in dl mode until the phone reboots. Goes back to the communication issue.
For problems/tips on connection issues, look HERE