If I have tension and the ball starts picking up the truck in park, I can usually just walk back and put the truck in neutral, then take my foot off the brake. Ball pops right off because it is already trying to pick up the truck.
I have also just eased the brake enough in reverse to get it to release. Don't have to release the brake all the way or give it any gas. That would be a bad idea! :)
Then back in park and lift the trailer the rest of the way to clear the ball completely.