If you're out fishing in the heat all day and the liver is not kept on ice it will get soft quickly. What I have done then is to take the livers and bake them on a rock,cement or best - black asphalt. Flip them several times and keep the juice in the tub(s).
Once the livers start getting leathery you can put them back into the rancid juice. They'll stay tough... Do not get this juice in cuts. Try to get as little as possible on your skin. Spill it on clothing and the smell never completely gets out.
But it's an awesome bait and stays on the hook well.
The video is long, but somewhere in it shows how it's done: http://www.youtube.com/watch?v=MfroQD2Y9ck