Nothing but blind luck.
We had 4 computers running at the house and we were hitting the green registration button like nobody business.
Whithin two minutes, Kim got a message saying the waiting room is full and I got a blank page.
After ten minutes of waiting with the blank page up (and following the advice of the twittersphere), I pressed the back button, press the green registration button and I got to the page asking me for a Last Name and Member ID.
Got our two 4 days (no preview) badges in 1 minute flat.
I understand why Comic-Con decided to use this method to sell badges instead of the at-con option; this is what I call "The Great Equilizer".
Both, first time attendees and hard-core veterans they all have the same chance to get a bagde, you just need a computer and a little bit of luck.