You need to anticipate and see this well before. Usually if you can try to escape the lip and just run down the wave or push through the back of the wave. I've seen some people just try and float over it and fail terribly
Once you are for sure that you wont make it, i spear the board out in front of me and do the pencil (arms and legs dead straight at your sides) straight down after stepping off the back. And if it is a beach break, I try and get as close to the sand/bottom as possible since the turbulance is lesser down there. But that is only if it is a huge wave. If it is reasonable size, I just dive head first through the bottom part of the inside wave face and it usually rolls over you. You just gotta dive hard. Some guys will superman dive out in front of the board, but I think its safer to step off the back. Sometimes with forward momentum its easier to flop forward, but you will get rolled that way. Just do the pencil and drop straight down.
P.S. All of these ideas can potential break your board as an end result. But this opinion was given based on the fact that you are very deep, in a very heavy, very large barrel. If this is not the case, save your board and sacrifice your body.