What gets me past the pain barrier? I would say the fact that I think about it all day long. When I am going to do heavy squats today I will play it over and over in my mind a successful lift. This prepares me for what is to come during training.
I also like to look around at the pathetic souls in the gym that are there to "work out" and watch their faces after I complete a real hard set, one in which these wussies would never even think to attempt. This is another motivator for me.
I also know that the pain barrier is where you make your money. So knowing this as I approach the difficult part of the set also keeps me going.
One last thing that keeps me going is the fact that I do not want to let myself down. I do not like to go to the gym to fail. This is not an option for me. I do have the days that I leave disappointed because I didn't meet my goal for the day, but I come back the next time with a vengance!
Oh yeah some really loud Metallica doesn't hurt either.